A general assumption made by the inverse theories is that lower level processing is enhanced in ASCs, and thus responsible for their superior performance on many tasks compared with individuals without ASCs. However, a recent line of research has revealed that at least some perceptual processes are adversely
affected in ASCs. Several studies have now shown that individuals with ASCs show higher thresholds for the perception of motion coherence (e.g. Spencer et al. 2000
; Milne et al. 2002
; Pellicano et al. 2005
). Two proposals have been advanced for this relative insensitivity to motion coherence. One is that higher levels of the dorsal visual stream, typically responsible for the integration of motion signals, are adversely affected in ASCs. The other is that motion integration difficulties result from deficits in early perceptual processes that drive the dorsal visual stream, in particular the magnocellular pathway. Insofar as inverse theories predict deficits in higher level processes and enhanced processing in lower level systems, then they propose that the difficulties in motion coherence observed in ASCs result from abnormalities in areas higher in the dorsal visual stream, such as area MT/V5.
We are currently assessing this assumption in a series of studies examining visual dorsal stream processing in ASCs. The emerging evidence suggests that, far from there being deficits at higher levels of processing in the dorsal visual stream, the causal deficit originates even before vision information reaches the visual cortex, in low levels of perceptual processing by magnocells in the thalamus.
For example, in one of our first experiments, we presented participants with a task designed to target selectively the information processed by magnocells or parvocells (Greenaway 2005
). Effective targeting of one or other of these two types of cell using behavioural psychophysical measures is notoriously difficult, owing to the often rather opaque relationship between individual cell responses and the visual system's overall response, the presence of a third broad class of cell in the LGN (‘koniocells’) and heterogeneity of response within each class of cell. Indeed, many previous studies of human observers have employed stimuli that should be preferred by individual magnocells, yet as Skottun (2000)
has demonstrated, such studies have not effectively measured either magnocellular or parvocellular function.
An elegant procedure that comes closest to targeting magnocellular functions is that developed by Pokorny & Smith (1997)
. It relies on the presence of robust differences in response to ‘luminance contrast’ between magnocells and parvocells. Luminance contrast is a measure of the magnitude of differences in light coming from different parts of a stimulus. At low contrast levels (faint stimuli), magnocells respond much more robustly than parvocells, but this response soon reaches a maximum. Parvocells' responses to low contrast stimuli are poor, but continue to increase as the contrast of the stimulus is increased. These two response properties give rise to two patterns of findings. Magnocells are more sensitive than parvocells to single low contrast stimuli, whereas parvocells are more sensitive than parvocells to differences between
higher contrast stimuli.
Pokorny & Smith's (1997)
procedure exploits this pattern of responses. On each trial, a ‘pedestal’ of four squares is presented. After looking at the pedestal for a while, one of the squares becomes momentarily slightly darker or lighter (b
). Because only one aspect of the display has changed, magnocells are very good at detecting even very faint changes in these stimuli (better than parvocells or indeed koniocells), so performance on this condition should be governed by how efficiently a person's magnocells are functioning. Accordingly, for our current purposes, we refer to this type of trial as the ‘magnocell condition’.
Typical display sequences for the study of (a) magnocellular and (b) parvocellular function.
In a second type of trial, the pedestal of four squares, one of which is slightly lighter or darker than the others, is presented simultaneously on a grey background (a). The observer must detect which of the squares is slightly darker or brighter than the others. Now, because all stimuli are presented at once, the task is effectively to distinguish between different levels of light in the four squares (rather than simply to detect a single light change). Accordingly, parvocells should govern performance under these conditions rather than magnocells.
We compared 17 children with ASCs and 17 neurotypical children, matched for chronological age (mean 12 years, range 9–14) and general mental functioning (mean raw Raven's matrices score 40). Each child's threshold was measured using a two-down, one-up staircase procedure (i.e. two correct responses led to a decrease in luminance increment and one incorrect response led to an increase in luminance increment). The task continued until 10 reversals had been reached, and the threshold was calculated by taking the mean of the last eight reversals.
-tests revealed that while the thresholds of the groups did not differ on the parvocell condition (t
=0.281), they did differ on the magnocell condition (t
=0.001). Thus, in comparison with the typically developing children, the children with ASCs exhibited clear deficits in the magnocell condition but no deficit or benefit in the parvocell condition. This finding, on the face of it, seems similar to findings of magnocellular dysfunction in other developmental disorders, such as dyslexia (e.g. Cornelissen et al. 1995
). However, a debate exists as to whether there are such deficits in other developmental disorders, because it is possible that the stimulus parameters chosen in previous studies may not be sensitive enough to adequately target magnocellular processing separately from parvocellular processing. We are currently extending this study examining magnocellular processing in ASCs using flicker stimuli that target magnocellular processing far more precisely than flicker stimuli used in previous studies (e.g. Pellicano & Gibson 2008
; see Skottun (2000)
and Plaisted & Davis (2005)
for discussions of the importance of appropriate stimulus selection in assessments of magnocellular dysfunction in discriminating developmental disorders). Further comparative research, using the kinds of procedure used here, is now urgently required to establish the degree of similarity of perceptual abnormalities between developmental disorders (Braddick et al. 2003
For our current purposes, however, this study demonstrates a perceptual difficulty that has no obvious benefit, and clearly does not compensate for lack of development of any higher order process in a straightforward inverse manner. Thus, although there are clear demonstrations of some superior processes that lead to highly skilled performance, there are other damaged processes that are deleterious to the individual and which require amelioration.