Abnormal responses to multi-element spatial stimuli in a subject with visual form agnosia
Alkhateeb W., Bromley JM., Humphreys GW., Javadnia A., Riddoch MJ., Ruddock KH.
1. We report measurements on a single patient, HJA, who has exhibited visual form agnosia since suffering a stroke. He has bilateral neural damage in the region of the occipital lobe served by the posterior cerebral artery. 2. He performs certain visual tasks normally; these include detection of depth in random dot stereograms, eye movements made in tracking moving targets, brightness matching between adjacent stimuli, identification of direction of movement and description of a number of visual illusions. 3. He has reduced sensitivity for detection of grating contrast and of an incremental flash, and he has no wavelength discrimination for small (2°) fields, viewed foveally. 4. We have recorded response times for his detection of a single target embedded in a number of identical, non-overlapping background elements. His responses for detection of a line which differs in orientation from the background lines are normal, which establishes that he can perform such search tasks. 5. His detection of a 2-D target element, distinguished from the background elements by orientation, magnification, texture, shape or luminance is in every case abnormally slow, and he requires, on average, ten times longer than normal for detection of such targets. 6. He frequently fails to detect 2-D targets, and the failure rates for different targets correlate closely with the response times required for their detection. Identification of 'null' stimuli which contain no target is, however, achieved with normal accuracy. 7. Measurements with different numbers of background elements establish that HJA suffers a specific loss in the processing of multi-element stimuli. He does not, however, exhibit 'tunnel' vision. 8. We argue that failure to process normally stimuli containing multiple elements is a critical feature of HJA's visual performance, and that it leads to disruption of his ability to recognise complex images. We discuss the neural correlates of his visual dysfunction.