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Does the modality in which information is presented influence how it is remembe- red? Human information tends to be organi- sed predominantly in either verbal or spatial
Psicothema, 1999. Vol. 11, nº 4, pp. 747- ISSN 0214 - 9915 CODEN PSOTEG Copyright © 1999 Psicothema
MEMORY IN TOUCH
Susanna Millar Oxford University
Models of short-term memory have to take into account that touch is not a tightly organised sin- gle modality. Touch, without vision or other external cues, depends on information from touch, move- ment and from body-centred (posture) cues. These inputs vary with the size and types of object, and with task demands. It is argued that the convergence and overlap of inputs from different sources is crucial to parsimonious organisation for memory and recall. «Modality-specific» input conditions thus form an integral part of the available information, which changes, and is changed by longer-term information. Three general principles apply: (i) Parsimony of coding modality-specific inputs for recognition and re- call; (ii) links with output systems which can «rehearse» information; and (iii) longer-term familiarity with procedures and types of coding. The introduction and first section discuss these points in relation to models for hearing and vision. The third section cites findings on modality-specific tactual memory, and explains tactual memory spans in terms of paucity versus redundancy of reference information to organise inputs spatially. Movements are considered next as inputs and as spatially organised outputs that can provide haptic rehearsal. The final section argues that intersensory modality-specific processes and longer-term memory need to be included as interrelated systems in STM models in order to account for memory in touch.
La memoria en el tacto. Los modelos de memoria a corto plazo deben tener en cuenta que el tac- to no es una modalidad única estrechamente organizada. El tacto sin visión u otras claves externas, de- pende de la información obtenida a partir del tacto, del movimiento y de las claves (la postura) centra- das en el cuerpo. Estos inputs varían con el tamaño y los tipos de objetos, y con las demandas de la ta- rea. La convergencia y el solapamiento de los inputs a partir de diferentes fuentes es crucial para la or- ganización de la memoria y el recuerdo. Las condiciones del input «específicas de la modalidad forman una parte integral de la información disponible, que cambia, y es cambiada por la información a largo plazo. Tres principios generales pueden aplicarse: (i) La parsimonia de la codificación de los inputs es- pecíficos de la modalidad para el reconocimiento y el recuerdo; (ii) las uniones con los sistemas del out- put que pueden «repetir» la información; y (iii) mayor familiaridad con los procedimientos y los tipos de codificación. En la introducción y en la primera sección se discuten estos puntos en relación con los modelos de visión y audición. La tercera sección cita resultados sobre la memoria del tacto específica de la modalidad, y explica la amplitud de la memoria táctil en función de la pobreza versus la redun- dancia de información de referencia para organizar los inputs de manera espacial. Después se conside- ran los movimientos como inputs y outputs organizados espacialmente que pueden proporcionar repeti- ción háptica. En la sección final se argumenta que los procesos intersensoriales específicos de la moda- lidad y la memoria a largo plazo necesitan ser incluidos como sistemas interrelacionados en los mode- los de MCP para poder explicar la memoria en el tacto.
Correspondencia: Susanna Millar Department of Experimental Psichology Oxford University South Parks Road OX1 3UD Oxford (United Kingdom)
form. Interestingly, each is connected with a particular sense modality: Space with vi- sion, language with hearing. Touch does not have such a well-defined link with a parti- cular form of knowledge. Feeling texture, temperature, pressure and pain in which touch is specialised, belong more to the «qualia» of experience —as do colour in vi- sion, or pitch in hearing—, than to specifi- cally linguistic or spatial forms of organi- sing information. The links between vision and space, hearing and language are not, of course, exclusive. Visual icons, scripts, signs and gestures can carry verbal informa- tion. Distance, direction, and even shape can be remembered from hearing sound pat- terns and verbal descriptions. Touch can convey both forms of information. Is there, therefore, any need to include the perceptual modality of inputs in models of memory? I shall argue that there is. The «medium» is not the «message». But the perceptual conditions in which inputs occur, and the links between perception and output systems, are an integral part of the type and amount of information that is available, and how that information is processed and re- membered. The general framework I prefer for des- cribing information processing and memory uses the metaphor of dynamic networks of interrelated and converging processes (Mi- llar, 1994, 1997). It focuses on active pro- cessing of the information that is available to the organism from external and internal sources. The contrast is with metaphors that imply a static architecture. Such metaphors underplay the continual, reciprocal influen- ce of previous and current information in processing, though that is obvious even in order effects in experimental presentations. Architectural metaphors suggest a rigid hie- rarchical («bottom-up» or «top-down») or- ganisation. Separate inputs from the moda- lities are either integrated at the top level by a separate translation mechanism, or simply
subserve a top-level abstract description that obtains for all inputs. Metaphors that imply active networks of interacting and converging processes accord better with cu- rrent findings on neural connections in the constantly active brain. Thus inputs from multisensory sources seem to converge in varying combinations in a number of brain areas. Above all, the metaphor of active, converging processes is needed for touch. Touch is not a single modality, nor even a single perceptual system. What we call «touch» refers to combinations of a number of different, converging sources of informa- tion that include inputs from some skin re- ceptors to greater or lesser extents. The no- tion of perceptual systems that receive in- puts from several sources is true also for other perceptual systems (Gibson, 1966). But in touch, the combinations of inputs vary with the type, size, meaning and fami- liarity of objects, as well as with current in- formation, and importantly also, with task demands. The questions we ask about me- mory in touch thus have to be seen in the context of the combinations of disparate in- puts that need to converge for recognition and short-term recall in different haptic con- ditions. Because differences in informational conditions are particularly important in touch, the present paper focuses on short- term memory for raised line configurations, rather than on object recognition. The neu- ral pathways in identifying objects (what?) and locations (where?) are not precisely the same (e.g. Anderson, 1987; Schneider, 1967; Weiskrantz, 1986). Recognising fa- miliar manipulable objects, that have well- known names, taps into a wide range of se- mantic knowledge and meaningful verbal descriptions of their every day use and even their visual appearance for subjects who ha- ve the experience. Prior knowledge has to be considered in examining short-term me- mory. But it is more circumscribed, and so-
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doff, 1989). But age differences in span are not eliminated by equating articulation ra- tes, as would be expected if speech output were the only factor (Henry & Millar, 1991, 1993). Familiarity with input sounds has significant effects on the size of immediate memory spans with age (Henry & Millar, 1991). Very young children tend not to use verbal rehearsal strategies (e.g. Conrad, 1971), but their memory span for sounds in- creases nevertheless to some extent. The contribution of longer-term memory to in- put coding for temporary memory is, there- fore, a further factor that has to be included in theoretical models (Henry & Millar, 1993). Familiarity with inputs, as well as links with output systems thus need to be considered in examining findings on touch. Experimental evidence for temporary memory for visual inputs has been even mo- re difficult to establish than for sounds. Vi- sion does not seem to have such an obvious closely-knit output system as vocalisation for hearing, though gestures in pointing, re- aching, drawing and locomotion constitute such output systems. However, the «percep- tion-action» models, deriving from Gibson (1979), which stress input-output links, use these to explain perceptual organisation wit- hout recourse to memory processes. In stu- dies of visual memory, on the other hand, the main controversy centred on the ques- tion whether visual memory is visual, or spatial and «abstract» in character. The vi- sual character of introspectively vivid vi- suo-spatial imagery seems obvious to those who experience it. Moreover, errors in «re- ading off» from very vivid or «eidetic» ima- gery show that it involves memory rather than persistence of «raw» sensory stimu- lation (Haber & Haber, 1964). Experimental findings have also been interpreted in ana- logy with a transitory quasi-«pictorial» vi- sual buffer that functions as if in «a me- dium» of coordinated space (e.g. Kosslyn, 1980, 1981). But experiments have not al-
ways distinguished between visual and spa- tial aspects of the materials. Discrepant vi- sual inputs that interfere with memory for visual presentations may do so because of their spatial form, and not because they are visual in character (e.g. Logie, 1986; Logie, Zucco & Baddeley, 1990). Thus, evidence that simple flashes of light have relatively little effect on memory for visuo-spatial in- formation has suggested that memory de- pends on «abstract» spatial representations. The notion implies that representations in memory are either not specifically visual in character (introspective reports are mista- ken), or that their visual character is func- tionally irrelevant, or both. Nevertheless, the total burden of current evidence suggests that temporary memory for modality-specific visual aspects, as well as for their spatial organisation, may be as- sumed. The fact that electrical stimulation of visual areas of the brain produces visual images (e.g. Dobelle, Mladejovsky & Gir- vin, 1974), and cognate evidence for the in- volvement of the visual cortex in attempts to form visual images (Goldenberg et al, 1990), suggests a physiological basis for «visual» aspects of introspectively reported imagery. Whether or not that is sufficient to establish that the visual character of repor- ted visual imagery is biologically useful, is a moot point. But further experimental stu- dies with children and adults also strengthen evidence for temporary memory for specifi- cally visual aspects, as well as for spatial in- formation (e.g. Logie & Baddeley, 1990; Logie & Pearson, 1997). Thus, recognition as well as recall for visual patterns was bet- ter than memory for sequences of move- ments to spatial targets, and the difference varied with age (Logie & Pearson, 1997). Taken together, the findings suggest a so- mewhat more complex picture of «working memory» than the original notion of short- term memory as a single system with one li- mited attention bottleneck.
MEMORY IN TOUCH
An alternative has been to assume com- pletely distributed storage systems with mul- tiple, separate «processing modules», each with its own temporary storage capacity (e.g. Monsell, 1984). In principle, modality- specific aspects of memory could easily be accommodated in such a system, though that was not the reason for the notion. The extre- me version assumes quite separate «modu- les» for different cognitive processes (Fodor, 1983). It was originally taken up with great alacrity, because the considerable functional specialisation of different areas of the brain seemed to accord better with descriptions in terms of separate «modules» than with the idea of one over-arching cognitive system. The evidence for regional cerebral speciali- sation of functions is not in doubt. However, the physiological and psychophysiological evidence neither requires, nor is actually compatible with the assumption that there are only very limited interactions between different brain regions (e.g. Squire, 1987). It can be argued, on the contrary, that the findings on specialisation of functions are better described in terms of the convergen- ce, in different combinations for different regions, of processing paths or activities ari- sing from specialised analyses. A more dy- namic picture of this kind seems to be re- quired particularly to account for evidence on memory processes in touch. Touch has been comparatively neglected in studies of psychology. The issues that ha- ve been considered here so far were raised predominantly by findings with visual and verbal materials. How do findings on touch fit in with these? Useful general theoretical models should account for findings on touch as well as on vision and hearing. But it is not sufficient to give theoretical des- criptions only at the most general level. The question is what forms of information can be assumed to be available for processing in tactual conditions, and what, if any, effects these have on memory.
The point is that «touch» is actually a euphemism for intersensory processing of information from a number of different sour- ces. The skin receptors convey information about texture, pressure, temperature and pain, as well as light touch, and the inputs occur in various combinations (e.g. Katz, 1925). Moreover, even passive touch can de- pend on concomitant input from propriocep- tive sources. Single unit recording has shown that discrimination of round and rec- tangular objects held in the monkey’s hand depends on converging inputs from touch re- ceptors in the palm and proprioceptive in- puts from the posture of the finger joints (Sa- kata & Iwamura, 1978). The organism quickly adapts to passive touch or pressure. Even passive perception usually needs inter- mittent, necessarily sequential, stimulation. Movement is crucial for active tactual ex- ploration to gain information about objects and environments (e.g. Gibson, 1962; Re- vesz, 1950). The term «haptics» was used to designate that proprioceptive and kinaesthe- tic inputs function together with touch in ex- ploring objects (Revesz, 1950). Gibson (1966) made the important point that the modalities of vision and hearing, as well as touch, are perceptual systems which depend on several sources of information. However, in touch or haptics the combi- nation of inputs from different sources does not seem to operate as an inherently tightly organised single perceptual system. Explo- ratory skill, experience with the stimulus ob- jects and with tasks, as well as physical cha- racteristics of stimulus objects determine haptic movements and performance (e.g. Appelle, 1991; Berla & Butterfield, 1977; Davidson, 1972). The combinations of con- verging information, from touch, finger, hand and arm movements, but also proprio- ceptive inputs from hand and/or body postu- res, differ not only with task demands, but also with the size, depth and composition of objects in touch (Millar, 1981b, 1994, 1997).
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dual difference factors, such as age (Millar; 1978 a,b). Thus, the same children who had recall spans of six or more tactual items that they could name, and improved further when the items were grouped, produced re- call spans of only two to three items for tac- tual nonsense shapes. If anything, grouping tactual nonsense shapes had the opposite ef- fect. The findings thus suggest that moda- lity-specific tactual memory must be assu- med, but it typically consists of only two to three tactual items. Prima facie, there seems to be no reason why memory spans for tactual patterns should be worse than for the same patterns in vision, if the tactual patterns are coded spatially as global shapes. Evidence for spa- tial coding comes from studies of brain in- sults in specialised cerebral regions that are known to be important in visuo-spatial or- ganisation (e.g. right hemisphere, post-pa- rietal area). Such infarcts also disrupt tem- porary memory for nonverbal tactual inputs in spatial tasks, although it is not always clear whether the loss is primarily sensory or attentional (e.g. Stein, 1991). But there is good evidence that spatial tasks involve a number of cortical and subcortical regions of the brain. The involvement of different regions (e.g. hippocampal, sensori-motor, somatosensory, pre-frontal, post-parietal, cerebellar, occipital and temporal) seems to depend on the combination of demands that tasks make on spatial, verbal and cognitive and memory skills, and on biomechanical constraints that particular conditions invol- ve. Spatial tasks generally show greater in- volvement of the right hemisphere, whether the stimulus materials are visual or tactile. However, it is not possible to make the re- verse inference, and to conclude that a left- hand advantage necessarily involves right hemisphere/spatial processing. Similarly, without additional evidence and adequate task analysis, a right hand advantage does not necessarily imply left hemisphere/ver-
bal processing. Thus, braille studies have reported left hand advantages, right hand advantages, and equal performance by both hands. Not surprisingly, the studies differ widely in the type of braille task that was being used, and in the experience and skill of the subjects who took part. Moreover, so- me apparently spatial tasks, such as aiming, have produced right hand advantages (Mi- llar, 1994, for review). To make sense, sti- mulus conditions, task demands, and sub- jects’ experience with exploratory strategies and materials have to be taken into account (e.g. Wilkinson & Carr, 1987). There has long been evidence that me- mory for unfamiliar tactual shapes is much less efficient than memory for the same un- familiar shapes in vision (e.g.; Berla & But- terfield, 1977; Gilson & Baddeley, 1969; Goodnow, 1971; Millar 1971a, 1977a, 1978b, 1990, 1991). Such assertions are of- ten misunderstood. It should be stressed at once, therefore, that they do not, of course, call in question that shape «can» be percei- ved by touch. That has long been establis- hed (e.g. Gibson, 1962; Katz, 1925), and re- quires no further research. Nor should evi- dence of poor tactual recognition be taken to mean that memory must necessarily be poor when the inputs are tactual. For instan- ce, tactual recognition of familiar three-di- mensional objects is often very good (e.g. Hatwell, 1978; Katz, 1925; Klatzky, Leder- man & Metzger, 1985; Weber, 1834, transl. 1978). Levels of efficiency, whether high or low, are of theoretical interest only because they raise questions about the factors that produce differences. The question here is thus about the informational conditions that underlie relatively small spans and poor me- mory for tactual inputs. Thus, it is relevant that poorer tactual re- cognition is more often found for unfamiliar two-dimensional raised line and raised-dot patterns, early in learning (e.g.; Goodnow, 1971; Hatwell, 1978; Lederman, Klatzky,
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Chataway & Summers, 1990; Millar, 1975a,b, 1977b, 1978a; 1985a, 1990, 1991), rather than for familiar objects (see earlier), and for three-dimensional shapes (David- son, Barnes & Mullen, 1974; Klatzky et al, 1985; Millar, 1974; Shimizu, Saida, & Shi- mura, 1993). The difference is not merely that solid forms, as such, necessarily provide more general information than outline shapes, as has sometimes been assumed. Many studies of 3-D shapes use familiar objects. For the recognition of such objects, shape (e .g. long versus round) is merely one among many possible cues, from temperature to re- silience to pressure, to the use, name and contextual meaning of the object (see ear- lier). The important advantage for recogni- sing the shape of unfamiliar 3-D versus 2-D forms is that 3-D shapes potentially afford more reference cues for spatial coding. Thus both hands are usually used to manipulate small three-dimensional forms. The two hands can act as spatial anchors and re- ference frames in relation to each other to locate component features. For large, statio- nary three-dimensional objects that are ex- plored by hand and arm movements, con- tour features can be located and spatially or- ganised by reference to body-centred re- ference frames. Two-dimensional raised li- ne configurations are usually explored by one finger, and, in particular if they are small, are difficult to relate to body-centred frames. We actually have sufficient evidence from past findings to be able to specify the main types of information that can vary in tasks that demand tactual memory for shape or form. Previous experience, both with and without vision, affects performance signifi- cantly. Modes of exploration and move- ments vary with familiarity, and with the ty- pe, size, depth and composition of stimuli. These, and task conditions are among the most important variables in haptic tasks
(Davidson, 1972, 1974; Davidson et al, 1974; Locher & Simmons, 1978; Millar, 1981, 1988a; Simmons & Locher, 1979). A crucial factor is the type, amount and con- vergence of reference information that is available from internal and external sour- ces, since it determines the spatial organisa- tion of shapes in haptic conditions (Berthoz, 1991; Millar 1981a, 1988, 1994; Paillard, 1991). Thus, the informational conditions in memory tasks for unfamiliar tactual raised- line and raised-dot patterns are typically characterised by a paucity of the reference cues that are needed for the patterns to be spatially organised as global shapes (Millar, 1978 a, 1988a, 1994, 1997). Braille patterns are an important exam- ple. The patterns lack salient features, be- cause all characters derive from a single, small (6.2 x 2 mm) matrix of six raised dots. The small size produces problems of acuity. But the principal difficulty does not lie in distinguishing patterns from each other. Discriminating characters is fairly easy even for the inexperienced (Katz, 1925; Mi- llar, 1977a,b). The main problem lies in the paucity of reference cues for coding the pat- terns as global shapes. The lack of redun- dancy means that there are no salient featu- res in the patterns that relate easily to each other. That makes it difficult to code the pat- tern as a distinctive global spatial configu- ration. It is also difficult to use self-referent frames to code the pattern as a global confi- guration, because the components are also too small to be related to body-centred re- ference cues to determine their position in the pattern. In principle, the tip of the ex- ploring finger could act as a frame in rela- tion to which dot locations could be deter- mined. However, that requires some expe- rience. Inexperienced people tend to «rub» over the dots too unsystematically to bene- fit from that. External reference cues are ty- pically lacking in blind conditions, unless
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tions also by touch. That usually requires prior experience. Direct evidence comes from filming the movements of the reading finger from underneath transparent surfaces (Millar, 1988 b, 1997)). Deliberate explora- tion of characters for shape information is seen typically in very slow, letter-by-letter readers, particularly by former print readers with extensive visuo-spatial experience, who learned braille relatively late in life (Millar, 1997). They move the exploring finger in up/down, zigzag and circular fas- hion over a character in attempting to cons- truct the total pattern. Such movements be- come much faster and stereotyped with ex- perience, without losing the typical charac- ter of scanning the form. For experienced readers, the manner of coding depends on task demands. Fluent braillists who have le- arned braille from the start, show evidence of coding letter shapes mainly in tasks that demand search for single characters. By contrast, fast reading for meaning is based on cues from dynamic shear patterns across the moving fingerpad (Millar, 1987). With experience, scanning movements by the two hands are organised, relative to each other, to provide the spatial information about the location of words and lines that is needed for reading texts, as well as the tactual (she- ar pattern) information which is translated rapidly into verbal (phonological and se- mantic) form for comprehension and recall. Taken together, the findings support the hypothesis that the small (two to three item) tactual memory spans for unfamiliar pat- terns (see earlier) can be explained by the paucity of reference information for coding inputs spatially in these conditions. Touch seems to be particularly good at feeling tex- ture differences. These can be used to en- hance discrimination (Millar, 1986 a ; Schiff & Isikow, 1966), and may underlie the mo- dality-specific tactual coding that was de- monstrated. Relying on texture differences in memory when reference information is
too sparse for spatial organisation would certainly be useful. But it would also ex- plain why non-verbal tactual memory spans were confined to two to three items. That was in contrast to large spans for inputs that are quickly re-coded verbally via long-term familiar names that can be rehearsed in the short term (see earlier). The hypothesis that spatial coding de- pends on the amount and redundancy of avai- lable reference information can also account for better recognition of 3-D forms that are not necessarily nameable. Such conditions exist, for instance, for experienced subjects who use exploratory strategies that actively seek environmental reference cues, or use the two hands relative to the body midline to act as self-referent frames in relation to which tactual «feels» can be localised. It should be noted that the term «frames of reference» is used here as an operatio- nally defined term. The relevant informa- tion can be experimentally manipulated. Blindfolding eliminates current information about environmental frames in relation to which the location of a stimulus can be de- termined. Such manipulation leaves body- centred reference frames intact. But self-re- ferent frames can also be enhanced or dis- turbed. Self-reference that is centred on the body midline can be disrupted by changing the posture or orientation of the body, or the orientation of displays (e.g. Millar, 1975c, 1985 b). Similarly, stimuli can be aligned to facilitate self-referent coding. We showed that positioning the two exploring fingers directly above the stimuli, in alignment with the body midaxis, produced a similar ad- vantage for symmetric over asymmetric (raised line) patterns to that which is found in vision (Ballesteros, Millar & Reales, 1998; Millar, Ballesteros & Reales, 1994). By contrast, shape symmetry is not an ef- fective cue in haptic conditions that afford few reference cues for spatial coding (Mi- llar, 1978 a). Exploring unfamiliar shapes
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blind with one finger, without special align- ment to the body-centred frames provides few references cues. In such conditions, symmetry has no advantage (Ballesteros, Millar & Reales, 1998, Millar, Ballesteros & Reales, 1994; Millar 1978 a). Reference frames in relation to which the location of tactual features can be determi- ned are needed for spatial coding. The hy- pothesis here is that spatially organised con- figurations reduce inputs, without loss of in- formation, to a form in which the informa- tion is manageable in memory over the short term. It predicts good memory for spa- tially organised tactual inputs in contrast to relying on relatively unorganised texture as- pects of tactual input. Studies concerned with nonverbal tactual stimuli have mainly used single stimulus configurations that are larger than braille patterns (e.g. Warm & Foulke, 1968; Locher & Simmons, 1978 ). The next section focu- ses more specifically on factors in memory for the somewhat larger movements that such layouts require.
Short-term memory for movements
Movements have mainly been studied as output systems. The focus has been on the kind of control (feedback, feed-forward, open-loop or closed-loop) that accounts for controls in sensorimotor and motor skills. However, recent work has emphasised the importance, as well as the diversity, of re- ference frames in relation to which move- ments are organised spatially (e.g. Berthoz, 1991; Jeannerod, 1988, 1991; Paillard, 1991). Most of the work has concerned mo- vements in visual environments in which the goal or target and other external cues were visible, either throughout the tests, or initially and at various points before blind- folding. A good deal of the evidence on spa- tial coding, for instance, of reaching move- ments, comes from studies with blindfolded
sighted subjects who were initially allowed sight of the target. The information subjects have at the beginning of the task was thus of the target location that could, in principle, be determined in relation to surrounding ex- ternal visuo-spatial frame cues, as well as relative to body-centred cues. When exter- nal frame cues are excluded by blindfolding subjects, the postural, body-centred frames that guide reaching and aiming movements sustain memory for the target location. In totally blind conditions, external environ- mental targets can be signalled initially by auditory or olfactory cues to which postures are adjusted. Short-term memory for blind movements within personal space (reach) has also been studied in the context of attentional or capa- city limitations (Laabs & Simmons, 1981). Laabs (1973) distinguished between coding location and movement or kinaesthetic in- formation by requiring recall of either the location or the extent of a positioning mo- vement, from a different position than in presentation. In such paradigms, the endlo- cation is recalled much more accurately than the extent of the movement. The fin- dings were widely interpreted to suggest that movements and spatial location are co- ded differently in short-term memory (Kel- so & Wallace, 1978; Kelso & Clarke, 1982; Laabs & Simmons, 1981; Marteniuk, 1978; Russell, 1976). Thus the endlocations of blind movements can be coded spatially by reference to body-centred spatial frames, while recall of movement extents depends on kinaesthetic inputs when these are not organised relative to spatial anchors or fra- mes. There has long been evidence that even unorganised movements survive in short-term recall. Thus, interpolating diffe- rent, irrelevant movements in delay periods, distorts recall of the target movement signi- ficantly (Adams & Djikstra, 1966). Recall of endlocations and of movement extents of positioning movements is not
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portant. Constant repetition of a given mo- vement distance interfered significantly with memory for endlocations of the move- ment when the starting position of these va- ried (Millar, 1985 b). Thus, with extensive practice, memory for movement extents can become extremely robust also. The whole question of effects of familiarity on short- term memory spans and on working me- mory is extremely important, not least for the recall and recognition of movement in- formation, and will be considered in more detail later. The point that needs to be stressed is that short-term memory for kinaesthetic inputs can be demonstrated even when these inputs are not spatially organised, although me- mory for unorganised kinaesthetic inputs (e.g. movement extent) is less accurate and less stable. We have shown modality-speci- fic motor memory in blind conditions from which any influence from longer-term vi- sual knowledge can be excluded (Millar & Ittyerah, 1991). Recall of a criterion move- ment by congenitally totally blind children showed signficant overshooting when an irrelevant larger movement was interpolated during the delay period, and significant un- dershooting of the criterion in recall when the interpolated irrelevant movement was shorter than the criterion. Blindfolded sigh- ted children showed the same effects. But in the case of the blind children it was also possible to exclude any possibility that the modality-specific effects on short-term me- mory could have been mediated by longer- term visual knowledge. Perhaps more important still, merely imagining the irrelevant movements during the delay period, without actually executing them, had similar effects on short-term re- call of a criterion movement as actually ex- cecuting the irrelevant movement (Millar & Ittyerah, 1991). Such movement imagery was shown also in conditions of total blind- ness. Congenitally totally blind children, as
well as blindfolded sighted children, were instructed to imagine («in their heads») exe- cuting an irrelevant shorter or longer move- ment during the delay before recalling the criterion, but not to make the movements overtly. Significant undershooting and overshooting in recall, depending on the ty- pe of irrelevant movement that had been imagined, was shown by the congenitally totally blind as well as by blindfolded sigh- ted children. The finding shows movement represen- tation in short-term memory. This is an im- portant finding, because it suggests a basis for mental rehearsal of movements even in conditions that exclude past as well as pre- sent visual information totally. Mental rehe- arsal of movements has long been shown to improve performance by sighted adults (e.g. Johnson, 1982), and is used in sports trai- ning. The effects of imagining irrelevant movements with blind children thus suggest that such strategies are, in principle, also available in totally blind conditions. Ins- tructions to mentally rehearse the criterion movement («imagine repeating the move- ment in your head») during delays signifi- cantly improved recall by the blindfolded sighted children. The improvement was in the same direction for the congenitally to- tally blind. But it did not reach significance level, despite the fact that discrepant move- ments during delays significantly interfered with their recall (see earlier). The lack of significant improvement is probably explai- ned by differences in informational condi- tions. All movements in the study had been designed to cross the body midline, in order to encourage movement coding by reducing spatial coding in terms of the body midline (Millar & Ittyerah, 1991). Young blind chil- dren tend to rely on memory for movements when coding in terms of self-referent spatial frames is made difficult or disrupted (Mi- llar, 1979, 1981b, 1985b, 1994). Memory for the criterion movement would be liable
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to disruption by the discrepant interpola- tions in delays if the criterion was coded in terms of kinaesthetic cues. But such coding may not be sufficiently economical to sus- tain active mental rehearsal to improve re- call. Rehearsal strategies are likely to be more efficient with codes that organise in- puts economically (see earlier). Thus the blindfolded sighted may be able to use ad- ditional reference frames, derived from vi- suo-spatial experience, to organise kinaest- hetic inputs when self-referent spatial co- ding is made difficult. Effective mental re- hearsal of kinaesthetic cues may depend on using more economical codes that reduce the memory load of kinaesthetic inputs. Spatial coding, either in terms of body-cen- tred or external frames, would produce bet- ter recall for that reason. Mental rehearsal of movements can be shown to have physiological correlates. Thus physiological studies of motor ima- gery have shown that patterns of activity in brain areas, including the motor cortex, are similar to patterns for actually executed mo- vements (Jeannerod & Decety, 1995). It may be important that subjects in most mo- vement studies are sighted people who are tested blindfolded. In some cases, testing occurs in the dark, but an external orienting cue remains visible throughout. It seems possible that in conditions of total blind- ness, efficient organisation of movements, which permits effective mental rehearsal, requires more practice, familiarity and con- sequently more influence of longer-term knowledge for mental rehearsal of move- ments to produce the same level of facilita- tion in short-term memory. That requires further study. The whole question of longer-term me- mory involvement in short-term and wor- king memory is extremely important, not le- ast for the recall and recognition of move- ment information. There is little doubt that immediate memory spans are larger for fa-
miliar than for new words, and for tactual patterns for which names can be retrieved easily and fast (e.g. Henry & Millar, 1991, 1993; Millar, 1975a). It seems likely that this is true also for immediate memory for movements. Many everyday movements are, of course, so well practiced that they run off automatically with great precision. A discussion of the feedback and feed-for- ward processes by which movement accu- racy is achieved is beyond the present brief. The point is rather that short-term motor memory in blind conditions can be based on kinaesthetic inputs, but that efficient mental rehearsal of movements, which facilitates recall, involves longer-term memory for ef- ficient means of organising the input infor- mation so as to reduce its memory load. The longer-term information that I have particularly in mind, as one basis of short- term memory for movement inputs, are practiced exploratory movements. The im- portance of input-output links for mental re- hearsal and the size of immediate memory spans has been mentioned before in connec- tion with memory for sounds (see, Henry & Millar, 1991, 1993). But there is also evi- dence suggesting that familiarity with well- organised exploratory movements form an important basis of tactual recognition me- mory. Thus, a surprising finding in getting con- genitally totally blind children to draw was that that even though they had never drawn before, the older subjects at least drew figu- res that differed little in general schema from those of their sighted cohorts (Millar, 1975 c). More surprising still, the blind chil- dren were much better at producing recog- nisable raised line drawings of the human figure than at recognising such figures (Mi- llar, 1986 b, 1990, 1991 a). For the sighted, the reverse is the case. Young sighted chil- dren can recognise drawings long before they can attempt to reproduce them. The finding for the blind should not, of course,
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Experimental studies of crossmodal, vi- sual/tactual performance have mainly used paradigms which dissociate the contributing modalities in spatial tasks. There is ample evidence from these that viewing the hand through distorting lenses alters subjects’ haptic perception of where their hands are (e.g. Howard & Templeton, 1966). Cross- modal coding of 3-D shapes has been de- monstrated with young children (e.g. Rudel & Teuber, 1964). But it involves more than one factor. Task conditions, even the order in which intramodal and crossmodal condi- tions are presented, have signficant effects (e.g. Millar, 1975 c). Fewer studies actually use simultaneous bimodal inputs. But it is fairly clear from the findings that concomi- tant inputs from different sources facilitate performance particularly in conditions in which the available task information is de- graded or insufficient. Thus adding infor- mation from touch to vision contributes lit- tle to bettering visual shape recognition, but adding visual information aids recognition of unfamiliar tactual shapes (Millar, 1971; Heller, 1982). Similarly, concordant inputs from texture and shape cues improve tactual performance, while discordant inputs dis- rupt recognition (Millar, 1986 c). The tasks that are of special interest here concern re- cognition and memory of relatively unfami- liar 2-D shapes and displays. Such tasks are rarely a problem in normal visual conditions even for very young children (e.g. Balleste- ros et al, 1998; Millar, 1971, 1972a; Millar et al, 1994). Visual conditions typically pro- vide the concomitant cues from different surfaces and features that act as reference frames for spatial coding from the start. Mo- reover, they usually overlap with conver- gent proprioceptive reference cues. The same tasks for which vision provides the most salient concomitant reference cues can also be performed with purely haptic in- formation. But the automatic overlap and redundancy of concomitant current referen-
ce cues from different sources is severely reduced the absence of sight. It is possible, in principle, to achieve the same levels of efficiency in haptic as in visual tasks that re- quire memory for 2-D shapes, distances, di- rections, or locations, as in vision, provided that alternative means of reference are avai- lable for spatial coding. At the same time, information conditions are not precisely the same for blindfolded sighted subjects, as for the congenitally totally blind who have no long-term visual experience (Millar, 1979, 1988 a, 1994). Body-centred reference fra- mes, prior procedural experience in sear- ching for external frame cues, and cue re- dundancy from alternative (e.g. hearing) sources become more important for spatial coding in these conditions. Memory for information from touch and movement, both in short-term blindfolding, and in the total absence of prior visual ex- perience, has been demonstrated. The prin- ciple of parsimony applies. Short-term spans for unfamiliar inputs are small. But they show effects of coding texture (shear pattern) in the case of shape tasks, and kina- esthetic coding in the case of unfamiliar movements, or both. Such memory coding is «modality-specific» in the sense that co- ding is derived from relatively specific as- pects of the input. More robust short-term haptic memory is found when the same haptic inputs are spa- tially organised. Such coding depends on accessible reference frame cues, as does spatial coding in visual conditions. The sa- me general principle thus applies. Neverthe- less, the informational conditions on which such coding depends differ from those in which visual cues are also present. Thus there is greater need to rely on body-centred reference information, or prior procedural knowledge, or to use alternative external (e.g. sound) location cues when that is pos- sible. Alternative means of more parsimo- nious coding can involve naming (see ear-
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lier), or counting. Moreover, the findings show that short-term haptic memory can in- volve mental rehearsal of movements. This differs from the articulatory coding that in- creases verbal memory spans. Memory even for the more organised haptic inputs thus includes information that derives from the haptic input conditions. Memory in touch thus includes «modality-specific» as- pects of the input information. To fit these findings into some form of «working memory» model, one might add a haptic - movement loop system, in ana- logy with articulatory coding. But to work effectively, the system would also need ac-
cess to longer-term visuo-spatial and/or egocentric reference information, as well as to longer-term procedural knowledge. Such access would need to be quite flexi- ble, especially in response to differences in task demands. It is possible that the as- sumption of a «central executive» in the system (Baddeley, 1990) might be suffi- cient to fulfill that role. But it is not quite clear how the model incorporates the con- tinual changes in longer-term memory that must be supposed with development and further experience, and which clearly play an important role, particularly in haptic short-term memory.
SUSANNA MILLAR
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