Second, it is now clear that several afferent visual processes contribute to the pupillary response. Recent studies have established that the human pupillary light reflex (i.e., changes in the pupil diameter in response to changes in light intensity) is driven by a light-sensitive pigment called melanopsin as well as by rod and cone photopigments (Gamlin et al.,
2007; Kawasaki & Kardon,
2007; Young & Kimura,
2008; McDougal & Gamlin,
2010). More importantly in the context of the present study, there is now good evidence for cortical contribution to the pupillary constriction to color, spatial pattern, and motion (Barbur & Forsyth,
1986; Barbur, Keenleyside, & Thomson,
1989; Heywood, Nicholas, LeMare, & Cowey,
1998; Weiskrantz, Cowey, & Le Mare,
1998; Wilhelm, Wilhelm, Moro, & Barbur,
2002; Barbur,
2004). When tested with hemianopic patients with occipital cortical lesions (postgeniculate lesions), pupillary color and pattern responses were either absent or much reduced to the stimuli presented to the blind hemifield, although the pupillary light reflex (i.e., pupillary response to intensity changes) was well preserved, particularly when the stimulus was of high contrast. Additionally, a lesion study with monkeys showed that the pupillary color response was abolished by removing the anterior inferotemporal cortex, while the pupillary light reflex was unaffected (Heywood et al.,
1998). Moreover, there is also evidence for selective loss of the pupillary light reflex. Wilhelm et al. (
2002) reported in patients with Parinaud's syndrome, which is caused by damage to the pretectal region in the dorsal midbrain, that the pupillary light reflex was almost abolished, while the pupillary color and pattern responses were little affected. In addition, Barbur, Wolf, and Lennie (
1998) argued that the level of visual processing at which different stimulus attributes are analyzed can be reflected in the latency of the pupillary response (i.e., the time measured from the stimulus onset to the response onset). Thus, the findings that the pupillary color response was 40 to 50 ms slower than the pupillary light reflex were considered to reflect processing delays arising in the cortex (see also Young & Alpern,
1980; Gamlin, Zhang, Harlow, & Barbur,
1998). These findings suggest a possibility of investigating subcortical and cortical contributions to the pupillary responses using the pupillary light and color responses, respectively.