Deficits in visual search for conjunctions of motion and form after parietal damage but with spared hMT+/V5

Abstract

It has been argued that area hMT+/V5 in humans acts as a motion filter, enabling targets defined by a conjunction of motion and form to be efficiently selected. We present data indicating that (a) damage to parietal cortex leads to a selective problem in processing motion–form conjunctions, and (b) that the presence of a structurally and functional intact hMT+/V5 is not sufficient for efficient search for motion–form conjunctions. We suggest that, in addition to motion-processing areas (e.g., hMT+/V5), the posterior parietal cortex is necessary for efficient search with motion–form conjunctions, so that damage to either brain region may bring about deficits in search. We discuss the results in terms of the involvement of the posterior parietal cortex in the top-down guidance of search or in the binding of motion and form information.

Keywords:

• Parietal cortex
• Feature binding
• Attention
• Visual search
• Motion

Acknowledgments

This research was supported by grants from the European Union and the MRC (Medical Research Council, UK).

Notes

¹ hMT+/V5 is the human analogue of the motion-sensitive middle temporal complex of the mace sometimes termed V5.

² At 57-cm viewing distance, 1 cm on screen corresponds to 1 degree of visual angle.

³ Interleaving trials in this manner means that each block is matched in terms of the different types of distractor that can occur, and the procedure also assists in overcoming potential order effects. While there may be costs associated with increased distractor uncertainty compared to pure single-feature blocks, these effects are likely to be very small.

⁴ There are constraints on matching accuracy by limiting the durations of moving displays because, if exposures become too short, then motion cannot be perceived. Here we varied luminance to avoid data limitations on motion processing.

⁵ Objective luminance in cd/m², where quoted, is based on the average of four readings taken with a Minolta LS110 photometer.

⁶ One consequence of matching the patients overall to the controls was that the patients tended to perform better than the controls for feature targets in their ipsilesional field. This is because only the patients showed an effect of field (indeed field was randomly determined for controls), being worse for contralesional targets. To equate performance overall then meant that the patients necessarily performed better than the controls for ipsilesional feature targets.