Basis Of Visual Perception Research Articles

Visual motion perception after brain damage: II. Deficits in form-from-motion perception

We investigated form-from-motion perception (FFM perception) in a sample of 39 patients with acquired brain damage. Pronounced FFM deficits were found in two patients (FM1 and FM2) with biparietal lesions. Both patients were able to identify the relevant figure, when it was not embedded in obstructive texture. Moreover, they could localize the figures in the FFM condition, although they could not reliably identify them. The two patients had normal motion coherence thresholds. Their performance in a static figure ground task did not differ from that of other patients. These findings imply that the FFM deficits are not caused by impairment of basic visual motion or form perception but are the consequence of damage to a parietal brain structure involved in the combined analysis of visual motion and form information. The nature and functional role of this brain structure as well as the implications of our results for models of FFM perception are discussed.

Neural Mechanisms of Visual Perception

It is more than half a century ago since Keffer H Hartline published his classical receptive fields studies of single optic nerve fibres in the frog. World War II intervened and the full impact of his work did not become apparent until the early fifties, when Horace Barlow extended Hartline's analysis in the frog and Stephen W Kuffler showed the on-centre and off-centre type ganglion cells in the cat retina. The next advances were made in the late fifties when Jerome Lettvin and Humberto Maturana described cells in the frog tectum with very complex response properties and when David Hubel and I discovered that cells in the cat striate cortex were sensitive to orientation of contours and binocular stimulation. Vision research has gone a long distance since that time—nonetheless we have just begun the long journey towards a detailed mechanistic understanding of the neural basis of visual perception. In this lecture I discuss the processing of visual information at the level of the striate cortex in the cat and monkey, and describe technical advances that have greatly facilitated the analysis of the neural mechanisms of visual perception.

Visuospatial Orientation in Parkinson's Disease

Visuospatial functioning in patients with Parkinson's disease was investigated using neuropsychological measures of basic visual perception, complex perceptual discrimination, and spatial orientation. Three subgroups of patients were described: (a) those with broadly impaired visuospatial abilities, (b) those with generally intact abilities, and (c) those whose performance on a task of spatial orientation was lower than their performance on a task of complex perceptual discrimination. These subgroup differences were also concordant with three other variables: age, duration of disease, and degree of dementia. It is suggested that decreases in spatial orientation functioning in Parkinson's disease may reflect the speed of progression of this disease.

Physiologic principles of studying visual functions

The retinal coding system for visual information comprises three important steps: 1) hyper- and depolarization of the sensory cells as the active basis for perception of light and dark; 2) reaction solely to changes in the bipolar cell on/off system, as a consequence of the action of the horizontal cells with the receptive fields, as a basis for adaptation to brightness and glare; and 3), under the influence of the amacrine cells, important time-space functions as a basis for visual perception of movement. The signal form, which changes over from analog to digital in the ganglion cell system, also results from this last step. therefore, both latencies and amplitudes (VEP) are important criteria for long signal response paths. Taking the examples of "statistical perimetry", visual acuity testing, and examination of contrast sensitivity, clinically relevant methodological problems from signal detection theory are illustrated: there are no "certain" measured values. This uncertainty and the age-dependence of normal values make it difficult to identify pathologic conditions, though trend analyses can help. As the sizes of the retinal areas examined by these methods differ so widely, correlations between the various examination methods are theoretically possible; but one method can never be substituted by another. In routine practice, to detect pathology early, one will sometimes deviate from the prescribed examination method (vision tests with low contrast for visual acuity).

What is the Stimulation for the Visual Perception of Object Motion?

Visual perception of object motion may derive from image displacements, eye movements, or configurational changes in the retinal image. Recent papers have suggested that (a) image displacements alone, and (b) image displacements in conjunction with real or intended eye movements, may be major stimuli for object motion perception. Both these suggestions face problems. For example, with regard to (a), image displacements in themselves are ambiguous; with regard to (b), motion perception during eye movements can be poor. On the basis of a selective review of induced movement, it is suggested that configurational changes in the retinal image form a more plausible basis for visual perception of object motion than either (a) or (b).