When does "no" really mean "yes"? A case study in unilateral visual neglect.

A patient with unilateral visual neglect indicated whether a dot was or was not present in a display. When present, the dot appeared equally often in the left and right visual fields. Although he typically denied having seen dots in his left visual field, he was able to make this judgment much more quickly than when no dot was in fact present. The mean response times when the dot was present (1135 and 1004 msec, for left and right) were almost twice as fast as the response times when no dot was present (2025 msec). This result suggests that the patient searched the visual fields individually, and in fact generated a "No" response based on detecting the dot in his neglected field. Thus, the mechanisms used to detect stimuli apparently are not rigidly linked to those used to classify them or to produce a response.

The judgement of absence in neglect.

The usual way of looking at neglect is by investigating how neglect patients fail to detect that something is there. In this study, we look at how neglect patients correctly detect that something is not there. Patients with parietal lesions (11 with and 16 without neglect) and 23 control subjects indicated whether a dot target was or was not present in a geometrical display. While control subjects were consistently (and unexpectedly) faster in the no-dot than in the dot condition, the distinguishing response time pattern of right parietal patients with neglect was not--as one might expect--a relatively longer response time to left vs right targets, but a longer response time to target absence vs presence. This may be due to a serial search or, alternatively, it might result from double-checking for target absence, produced by lowered perceptual confidence. Since this "wariness" about stimulus absence seems to operate in parallel with neglect patients' denial of the deficit, we conclude that the response time pattern observed in this study could be used as a measure of subjective (un)awareness of neglect.

Visual processing in migraineurs.

Twelve migraine subjects with aura and 12 matched control subjects performed four computerized visual tasks. Because chronic electroencephalographic and regional cerebral blood flow abnormalities in posterior brain functions have been documented during interictal periods, migraine subjects were tested between migraine attacks. Two tasks, orientation detection and temporal order judgement, were devised to examine 'low-level' visual processes. The results were compared with results from two tasks, picture naming and word priming, that were devised to examine 'high-level' visual processes. Regarding the response time data across all four tasks, the migraine group was faster in the two low-level tasks. There were no differences in error rate in any of the four tasks. The migraineur's apparent response time advantage in the low-level tasks provides a psychophysical corroboration of their assumed oversensitivity to visual stimuli. The remaining two tasks, picture naming and word priming, which involve the use of previously stored information, do not distinguish migraineurs from matched control subjects. These results suggest that migraineurs are better in low-level visual processing, in that the signals to the primary visual cortex (Area V1) are processed more rapidly, but this hypersensitivity does not carry over to the subsequent processing stages (beyond V1 to superior parietal and inferior temporal cortices), as evidenced by the absence of a response time advantage in the two higher-level vision tasks.

Field-specific deficits in visual imagery following hemianopia due to unilateral occipital infarcts.

Patients with stable, homonymous hemianopia due to unilateral occipital infarcts and control subjects performed a task in which they judged whether or not an arrow was pointing at one of the dots in a pattern of dots they had recently seen in free vision, but was no longer visible. This task, as shown in prior studies as well as in the present one, involves the use of visual imagery. The patients made more errors when the arrow pointed to the side ipsilateral to their hemianopia than they did when the arrow pointed to the side contralateral to their hemianopia. The patients' performance in control tasks indicated that this impairment was not due to deficits in several non-imaginal processes required to perform the imagery task, namely scanning the dots and perceiving their spatial positions, memory for their position and encoding arrow direction. These findings support the view that visual imagery involves topographically organized visual areas of the occipital lobe.