Visual areas and spatial summation in human visual cortex

Functional MRI measurements can securely partition the human posterior occipital lobe into retinotopically organized visual areas (V1, V2 and V3) with experiments that last only 30 min. Methods for identifying functional areas in the dorsal and ventral aspect of the human occipital cortex, however, have not achieved this level of precision; in fact, different laboratories have produced inconsistent reports concerning the visual areas in dorsal and ventral occipital lobe. We report four findings concerning the visual representation in dorsal regions of occipital cortex. First, cortex near area V3A contains a central field representation that is distinct from the foveal representation at the confluence of areas V1, V2 and V3. Second, adjacent to V3A there is a second visual area, V3B, which represents both the upper and lower quadrants. The central representation in V3B appears to merge with that of V3A, much as the central representations of V1/2/3 come together on the lateral margin of the posterior pole. Third, there is yet another dorsal representation of the central visual field. This representation falls in area V7, which includes a representation of both the upper and lower quadrants of the visual field. Fourth, based on visual field and spatial summation measurements, it appears that the receptive field properties of neurons in area V7 differ from those in areas V3A and V3B.

Perceived speed of colored stimuli

The influence of contrast and color on perceived motion was measured using a speed-matching task. Observers adjusted the speed of an L cone contrast pattern to match that of a variety of colored test patterns. The dependence of speed on test contrast was the same for all test colors measured, differing only by a sensitivity factor. This result suggests that the reduced apparent speed of low contrast targets and certain colored targets is caused by a common cortical mechanism. The cone contrast levels that equate perceived speed differ substantially from those that equate visibility. This result suggests that the neural mechanisms governing speed perception and visibility differ. Perceived speed differences caused by variations in color can be explained by color responses that are characteristic of motion-selective cortex.

Dichotic pitch: a new stimulus distinguishes normal and dyslexic auditory function

Two patterns of appropriately filtered acoustic white noise can be binaurally fused by the human auditory system to extract pitch and location information that is not available to either ear alone. This phenomenon is called dichotic pitch. Here we present a new method for generating more effective and useful dichotic pitch stimuli. These novel stimuli allow the psychophysical assessment of dichotic pitch detection thresholds. We show that dichotic pitch detection is significantly impaired in individuals with developmental dyslexia, as compared to average readers. These results suggest a low-level auditory deficit associated with dyslexia and also demonstrate the potential value of our new dichotic pitch stimuli for assessment of auditory processing.

A covariance structure analysis of flicker sensitivity

We tested whether linear structural models of the mechanisms underlying flicker sensitivity could reproduce the variance-covariance matrix of temporal contrast sensitivity data. Monocular sensitivities to frequencies between 2.5 and 45 Hz were measured for 124 subjects, ages 18-88 yr. Exploratory factor analyses revealed that both a two-mechanism and a three-mechanism model could adequately account for the data. Furthermore, confirmatory factor analyses and full structural equation models, using age as an explanatory variable, supported both models, with the three-factor model giving a somewhat better representation of the data. Parsimony favors the two-mechanism model. But patterns of loss associated with pre-exudative age-related maculopathy are more easily understood in terms of three underlying mechanisms.

Flicker sensitivity and fundus appearance in pre-exudative age-related maculopathy

PURPOSE

To evaluate whether foveal flicker sensitivity and fundus appearance are good predictors of exudative age-related maculopathy (ARM) when the effects of aging, retinal illuminance, and criterion differences are controlled.

METHODS

Fellow eyes of monocular exudative ARM patients were tested at baseline. Seven of these eyes have now developed exudative ARM. Therefore, at baseline they were in pre-exudative stages of ARM. The foveal flicker sensitivity and fundus appearance of the pre-exudative and nonconverted eyes were compared with healthy, age-matched eyes. The flicker stimulus was a uniform, 2.8 deg circular field at 660 nm, modulated sinusoidally at frequencies from 2.5 to 50 Hz. Fundus photographs were evaluated using the Wisconsin ARM grading system.

RESULTS

Flicker modulation sensitivity at two frequencies discriminated pre-exudative from healthy older eyes with 100% accuracy. Using the same criterion, pre-exudative eyes also were discriminated from nonconverted eyes with 100% accuracy. Whereas an overall fundus ARM risk score discriminated pre-exudative from healthy older eyes with 100% accuracy, it did not discriminate pre-exudative from nonconverted eyes at better than chance levels.

CONCLUSIONS

There were functional changes in the retina preceding development of exudative ARM. Foveal flicker sensitivity at low- to mid-temporal frequencies seemed highly sensitive to these pre-exudative changes in this relatively small group of subjects. The authors hypothesize that foveal flicker sensitivity is a good predictor of exudative ARM and a sensitive monitor of retinal function in pre-exudative ARM. These predictions are being tested on a larger, independent sample.