One of the most famous books written by Oliver Sacks, popular author and beloved behavioral neurologist, is The Man Who Mistook His Wife for a Hat. One of the chapters describes the case of a patient with visual agnosia, or the inability to recognize objects.
Below is a conversation between Sacks and Dr. P, the patient with visual agnosia.
I showed him the cover [of a National Geographic Magazine], an unbroken expanse of Sahara dunes.'What do you see here?' I asked.'I see a river,' he said. 'And a little guest-house with its terrace on the water. People are dining out on the terrace. I see coloured parasols here and there.' He was looking, if it was 'looking', right off the cover into mid-air and confabulating nonexistent features, as if the absence of features in the actual picture had driven him to imagine the river and the terrace and the colored parasols.I must have looked aghast, but he seemed to think he had done rather well. There was a hint of a smile on his face. He also appeared to have decided that the examination was over and started to look around for his hat He reached out his hand and took hold of his wife's head, tried to lift it off, to put it on. He had apparently mistaken his wife for a hat! His wife looked as if she was used to such things.
Visual agnosia is caused by an acquired brain injury to high-level object processing areas in lateral occipital and ventral temporal cortices. Primary and secondary visual regions are spared, meaning that basic visual responses are not compromised. Language and naming are intact, as is the ability to identify objects through other modalities (e.g., auditory, tactile).
A case study published in Neuron (Konen et al., 2011) describes a patient similar to Dr. P. Patient SM is a right-handed, 36 year old male who sustained a closed head injury in an automobile accident at the age of 18. He recovered after the accident but was left with visual agnosia and prosopagnosia, an impairment in recognizing faces. The damaged area of his brain was fairly circumscribed1 and smaller in size than in many other patients with visual agnosia:
The lesion was situated within LOC, anterior to hV4 and dorsolateral to VO1/2, and was confined to a circumscribed region in the posterior part of the lateral fusiform gyrus in the RH [right hemisphere]. Typically, this region responds more to intact objects than scrambled objects and damage to this circumscribed area is likely the principle etiology of SM's object agnosia.
Figure 4 (modified from Konen et al., 2011). Lesion Site of SM in Anatomical Space. (C) Axial view of the lesion site marked in green. The slices were cut along the temporal poles for enlarged representation of occipitotemporal cortex.
In addition, detailed topographic mapping of visual cortex was conducted using fMRI in SM and controls. Responses in early cortical areas (prior to the lesioned fusiform gyrus in the feedforward processing stream) were intact in SM.
Figure 1 (Konen et al., 2011). Topographically Organized Areas and Lesion Site in SM (A) and Control Subject C1 (B). Flattened surface reconstructions of early and ventral visual cortex. The color code indicates the phase of the fMRI response and region of visual field to which underlying neurons responded best. Retinotopic mapping revealed regular patterns of phase reversals in both hemispheres of SM that were similar to healthy subjects such as C1. SM's lesion is shown in black, located anterior to hV4 and dorsolateral to VO1/2. LH = left hemisphere; RH = right hemisphere.
Conversely, the hemodynamic response to object presentation was reduced in the area surrounding the lesion, as expected. But the most remarkable and surprising aspect of the study is that reductions in object-related responses were also observed in the corresponding region of SM's intact left hemisphere. How might this be explained?
...while the RH lesion might be primary, this lesion has remote and widespread consequences, with functional inhibition of homologous regions in the structurally intact hemisphere. Such a pattern raises the question whether the observed brain-behavior correspondence serves as the neural underpinning of the impairment or whether reconceptualizing SM's agnosia in terms of disruption to an interconnected more distributed neural system might be a better characterization of SM's pattern and of agnosia more generally.The authors discuss their findings in the video below, where Marlene Behrmann mentions that SM mistook a picture of a harmonica for a cash register.
Video Abstract (mp4)
Highlights► Unilateral lesion of lateral fusiform gyrus in right hemisphere causes object agnosia ► Agnosic patient exhibits normal retinotopy and visual responsivity in visual cortex ► Object-responsive and object-selective responses are reduced in both hemispheres ► Cortical plasticity evident with reorganization of intermediate and higher-order areas
1 The Methods section notes additional damage in the corpus callosum and left basal ganglia.
Konen, C., Behrmann, M., Nishimura, M., & Kastner, S. (2011). The Functional Neuroanatomy of Object Agnosia: A Case Study. Neuron, 71 (1), 49-60 DOI: 10.1016/j.neuron.2011.05.030
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