Lucia M. Vaina
Brain and Vision Research Laboratory
Boston University

The psychophysical and neurophysiological demonstrations of sensitivity to radial flow patterns have been taken as powerful evidence that optic flow guides locomotion (for a review see [1]). However, recent data from two stroke patients with bilateral parietal lobe infarcts supports the view that optic flow may not be necessary for guiding locomotion [2]. A recent study form our laboratory revealed that patients who are unable to discriminate radial motion patterns or heading from optic flow alone (RDK stimuli), could successfully navigate in real-world environments and could correctly perceive their heading in simulated visual scenes containing clearly identifiable landmarks.

The behavior and psychophysical performance of such patients is problematic for the velocity field account of heading. This prompted us to carry out an fMRI study of normal subjects and patients impaired on heading perception from motion cues alone [3,4]. We compared cortical activity elicited by two tasks of heading discrimination. In one, Heading-RDK, the only cue was visual motion, while in the second, Heading-Room, landmarks provided an additional spatial cue. The results showed that (a) in normal subjects there was no significant difference between the cortical loci of activity in the two tasks,. Both tasks activated the lingual gyrus, and the inferior and superior parietal lobule. These areas were not at all, or only slightly active in the patient; and (b) the pattern of cortical activity in the patient unable to discriminate flow-based heading, was most significant in the superior temporal gyrus on the left and in the parahipoccampal gyrus in the hemisphere contralateral to her lesion These regions were not activated in the normal control subjects. Interestingly, normal controls showed activity in these regions during the acquisition of spatial layout for navigation tasks as shown by Gron [5] and Aguirre and d’Esposito [6].

Our fMRI study suggests that, when optic flow information is available it used in computing one’s heading. However, alternative strategies can be used when other cues are available (and perhaps optic flow cues are not). One such strategy is to use the spatial layout of the environment.

[1] Warren W (1999). Visually Controlled Locomotion, 40 Years Later. Ecological Psychology, 10(3-4): p. 177-219.

[2] Vaina LM, et al. (1999). Impaired complex motion perception in patients with bilateral posterior parietal lesions. Investigative Ophthalmolology in Vision Science, 40(4): p. S765. Abstract nbr 4042.

[3] Vaina LM, Solovyev S, Kopcik M, Chowdhury SF, (2000). Impaired self-motion perception from optic flow: A psychophysical and fMRI study of a patient with a left occipital lobe lesion. Society for Neuroscience Abstracts, 26(1): p 1065. Abstract nbr 399.11.

[4] Vaina LM, Solovyev S, Botnar O, Kopcik M. (2001). Perception of heading: Is optic flow necessary? Perception Suppl. v30.

[5] Gron G, et al. (2000). Brain activation during human navigation: gender-different neural network as substrate of performance. Nature Neuroscience, 3(4): p. 404-408.

[6] Aguirre GK, Zarahn E, and D'Esposito M, (1998). Neural components of topographical representation. Proc. Natl. Acad. Sci. USA, 95(3): p. 839-46