Dissociations Between Perception and Imagery

To what extent do mental imagery and visual perception rely on the same neural machinery? Although early data suggested that imagery and perception were not neurally dissociable (Farah, 2000), suggesting that they may rely on nearly identical information processing components in the brain (as discussed in yesterday's post), more recent cases have illustrated a variety of ways in which the two functions can be selectively impaired. For example, some patients are able to draw an object from memory, but moments later are unable to recognize their own drawing.

How would one diagnose a problem with imagery as distinct from a problem with visual perception, or "object knowledge" more generally? The approach used by Farah with patient RM was to administer a sentence verification task in which half the sentences required mental imagery. For example, "A grapefruit is larger than a cantaloupe" would require verification by RM. Since it's unlikely that long-term associative memory contains this type of detailed information, one would have to engage in some kind of mental visual comparison. The other sentences to which his responses were compared did not require imagery, such as "The US government functions under a two-party system." Pretesting on normal subjects was used to equate these two sentence types on difficulty. RM performed poorly on the imagery questions, but did well on all other sentence verifications.

RM also underwent tests to verify that his perceptual and visual abilities were still intact. RM could name objects in pictures, could copy drawings nearly perfectly, but refused to complete the part of the test in which he was required to copy a drawing from memory. Likewise, when tested for his recognition of colors, he performed nearly perfectly, but was unable to declare the typical color of objects from memory.

Patient HJA (Humphreys & Riddoch, 1987) shows the opposite pattern; if asked to draw an object from memory, HJA could complete a realistic drawing, but was unable to recognize these objects in real life or answer imagery questions (e.g., "which has more red in it, a plum or an eggplant?"). Interestingly, as the years wore on, HJA lost this ability too, suggesting that visual semantic memory may require maintainence, or continual input from the environment, to remain accessible. Patient MD (Bartolomeo, 1998) showed a similar pattern of dissociations, in that she was impaired at identifying visual objects, but was perfectly able to answer questions involving mental imagery. Furthermore, she was frustrated that the researchers kept asking questions about her imagery abilities, which she emphatically claimed were fine!

Although these stories are fun to tell and fascinating to imagine, three points justify skepticism of these reports:
  • First, these dissociations could be caused by a “disconnection” syndrome, in which afferent perceptual inputs to a shared imagery/pattern recognition system would be selectively damaged without concomitant damage to its efferent inputs (Farah, 2000).
  • Second, many of the patients also sustained fairly broad impairments (including optic aphasia, prosopagnosia, and pure alexia; Bartolomeo, 2002), so their behavioral results should be interpreted as reflecting imagery problems only with caution.
  • Third, the majority of cases reported before 1988 support the idea that imagery and pattern recognition are served by the same regions (Bartolomeo, 2002) and are quite rare.

Kosslyn & Thompson were able to identify the precise conditions under which perceptual primary visual area (PVA) regions are activated by imagery (2003). Asking subjects to visualize high-resolution details, requesting that those mental images be non-spatial, and using high-resolution neuroimaging techniques were all associated with increased PVA activity, and could together explain most variation in the existing PVA-neuroimaging data. To summarize, imagery may be anatomically distinct from pattern recognition only insofar as it deals with spatial, or low-resolution images.

Therefore, an interesting question is whether those patients showing selective impairments of perception and not visual imagery actually sustained damage to regions prior to primary visual cortex, or if perhaps they showed individual differences in the degree to which they activate PVA for detailed imagery. Nonetheless, this data poses a problem for Kosslyn's information processing approach to mental imagery: just because it appears to us that the same anatomical regions would be capable of supporting both mental imagery and perception, we should not assume that this is the case. As structural imaging gains resolution (such as dtMRI) we should be able to determine whether the double dissociations observed by Bartolomeo are in fact "disconnection" syndromes, or whether Kosslyn's architecture truly needs revision.

One clear theoretical problem lingers, however. According to Kosslyn & Thompson’s analysis, relatively simple, non-spatial mental imagery may actually fail to elicit PVA activity. If this activity is truly absent (as opposed to immeasurably small), then what kinds of representations are activated? Tomorrow's post will address the differences in the representational format of imagery and perception (or lack thereof?), using evidence from visual illusion research.

Related Posts:
An Information Processing Approach to Mental Imagery


Blogger Dan Dright said...

Chris, these posts are absolutely fabulous. This is one of my pet fave areas for active research and idle thought (the good sort) and I really appreciate your spot-on critique.

Nice work.

3/18/2006 07:56:00 PM  

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