Facilitation of pattern recognition by cuing foveation with the luminance centroid as origin of the frame of reference

Relating spatial and temporal orientation pooling to population decoding solutions in human vision

Spatial pooling is often considered synonymous with averaging (or other statistical combinations) of local information contained within a complex visual image. We have recently shown, however, that spatial pooling of motion signals is better characterized in terms of optimal decoding of neuronal populations rather than image statistics (Webb et al., 2007). Here we ask which computations guide the spatial and temporal pooling of local orientation signals in human vision. The observers’ task was to discriminate which of two texture patterns had a more clockwise global orientation. Standard textures had a common orientation; comparison textures were chosen independently from a skewed (asymmetrical) probability distribution with distinct spatial or temporal statistics. We simulated observers’ performance using different estimators (vector average, winner-takes-all and maximum likelihood) to decode the orientation-tuned activity of a population of model neurons. Our results revealed that the perceived global orientation of texture patterns coincided with the mean (or vector average read-out) of orientation signals accumulated over both space and time. To reconcile these results with our previous work on direction pooling, we varied stimulus duration. Perceived global orientation was accurately predicted by a vector average read-out of orientation signals at relatively short stimulus durations and maximum likelihood read-out at longer durations. Moreover, decreasing the luminance contrast of texture patterns increased the duration of the transition from a vector average to maximum likelihood read-out. Our results suggest that direction and orientation pooling use similar probabilistic read-out strategies when sufficient time is available.

Methylphenidate effect on hemispheric attentional imbalance in patients with traumatic brain injury: a psychophysical study

PRIMARY OBJECTIVE

We found spatially asymmetric allocation of attention in patients with traumatic brain injury (TBI) without overt asymmetry on neurological examination. The possible effect of Methylphenidate in reducing this asymmetry is evaluated in the current research.

RESEARCH DESIGN

Psychophysical study using a visual spatial attention task. Identification rates were measured after precuing attention to different visual field loci. TBI patients were tested before, during and after administration of Methylphenidate.

METHODS AND PROCEDURES

After precuing to a locus 5 degrees into the left or right hemifield, target patterns were presented briefly at the cued location (valid), or on the opposite side (invalid) - requiring an attentional shift. Patients were treated with a gradually increasing dosage of a psychostimulant over two weeks, followed by a similar two-week period of gradually diminishing dosage. Patients were tested before treatment, at its peak, and (twice) following its completion.

MAIN OUTCOME AND RESULTS

Patients demonstrated significantly worse performance with leftward than with rightward cross-hemifield shifts of attention. This difference was significantly reduced during and following treatment.

CONCLUSIONS

Asymmetric performance and improvement with treatment suggest that diffuse TBI damage leads to a lateralized attention-related deficit. These findings support the hypotheses that attention is a distributed and asymmetrically lateralized function. The findings are consistent with the conclusion that Methylphenidate may be an effective treatment for attentional deficits in TBI patients.

Hemispheric visual atentional imbalance in patients with traumatic brain injury

We find a spatially asymmetric allocation of attention in patients with traumatic brain injury (TBI) despite the lack of obvious asymmetry in neurological indicators. Identification performance was measured for simple spatial patterns presented briefly to a locus 5 degrees into the left or right hemifield, after precuing attention to the same (ipsilateral) or opposite (cross-hemifield) side. Though the cue was non-predictive of target location overall, performance was significantly slower for cross than for ipsilateral trials in both patients and controls. We tested 21 TBI patients without overt focal brain damage and nine control subjects. Only patients demonstrated significantly worse performance for left side presentation in the ipsilateral condition. Furthermore, in the cross-hemifield condition, the left-right difference seen in TBI patients was significantly larger-reflecting a failure in producing a leftward attention shift. Again no significant difference was found in controls. These hemifield effects suggest an asymmetry in the ability of TBI patients in shifting attention to the left hemifield, whether from central fixation or from a cue in the contra-lateral hemifield. The results support basic hypotheses regarding visual attention: Attentional control may be asymmetric and attention may be a distributed, rather than localized cortical function.

Coordinate Frame for Pattern Recognition in Unilateral Spatial Neglect

The present research examines the effect of spatial (object-centered) attentional constraints on pattern recognition. Four normal subjects and two right-hemisphere-damaged patients with left visual neglect participated in the study. Small, letterlike, prelearned patterns served as stimuli. Short exposure time prevented overt scanpaths during stimulus presentation. Attention was attracted to a central (midsagittal) hation point by precuing this location prior to each stimulus presentation. Minute (up to 1.5° of visual angle) rightward and leftward stimulus shifts caused attention to be allocated each time to a different location on the object space, while remaining in a fixed central position in viewercentered coordinates. The task was to decide which of several prelearned patterns was presented in each trial. In the normal subjects, best performance was achieved when the luminance centroid (LC; derived from the analysis of low-spatial frequencies in the object space) of each pattern coincided with the spatial position of the precue. In contrast, the patients with neglect showed optimal recognition performance when precuing attracted attention to locations within the object space, to the left of the LC. The normal performance suggests that the LC may serve as a center of gravity for attention allocation during pattern recognition. This point seems to be the target location where focal attention is normally directed, following a primary global analysis based on the low spatial frequencies. Thus, the LC of a simple pattern may serve as the origin point for an object-centered-coordiate-frame (OCCF), dividing it into right and left. This, in turn, serves to create a prototype description of the pattern, in its own coordinates, in memory, to be addressed during subsequent recognition tasks. The best match of the percept with the stored description may explain the observed advantage of allocating attention to the LC. The performance of the brain- damaged patients can be explained in terms of neglect operating in the OCCE