A study of pupil responses in a subject with damaged primary visual cortex
Barbur JL., Cole VA., Sahraie A., Simmons A., Weiskrantz L., Williams SCR.
Purpose In addition to pupil response s to increments in light flux on the retina, recent studies have also demonstrated the existence of Pupil Colour Responses (PCR), pupil grating responses and pu jil motion responses. The purpose of the study was to establish how damage to the primary visual cortex in man affects the PCR component. Methods PCRs ha\ e been measured in normal subjects and in patients with damaged primary visual cortex using isoluminant chromatic stimuli. In order to isolate chromatic signals wher saturated colours are employed, the stimuli were generated in an array of checks c efined by random luminance contrast. The mean luminance of the test stimulus also changed randomly every 50 ms, within a range specified as a % of background uminance. In addition to pupillometric studies, functional magnetic resonance imaging (fMRI) of the brain was also carried out using the same stimuli. Results The -uminance masking techniques employed fail to eliminate PCRs even when larger anplitude pupil responses elicited by achromatic luminance increments are -îliminated completely. Large chromatic stimuli presented to the subject's cortically blind hemifield, can generate both increased brain activity (as revealed b) fMRI) and significant pupil colour responses, particularly for chromatic modulations towards the red region of the spectrum locus. The superior colliculus on the side of the lesion is activated and this suggests the involvement of a sub-cortical route. Si jnificant activation of several areas of the brain in the intact hemisphere is also ooserved. Conclusions Residual PCRs that are restricted to chromatic modulation? towards the red region of the spectrum locus can be elicited under stimulus conditions that isolate chromatic signals even in the absence of V1. The presence or abser ce of significant brain activation by such stimuli correlates well with the observed pupil responses.