Limits and Localisation of Speed Perception: an investigation of the processing of object speed by the human visual system.

The ability of an organism to estimate the speed of moving objects in the surrounding environment provides an important means by which it can obtain information about the nature of the outside world. The analysis of speed is important because it provides the basis for enabling organisms to track, catch or even avoid moving objects around it, tasks which make the perception of speed vital to an organism's maintained survival. This research aims to improve our basic understanding about how the human brain and visual system accomplish something as fundamental as the analysis of the speed. It has only been relatively recently that a picture has started to emerge as to exactly how, and in which parts of the brain, the speed of moving objects is analysed. Yet despite growing insight there remain many gaps in our knowledge. Two fundamental questions that have been largely unexplored are: 1) How are different kinds of motion cue used in the perception of speed? Movement of objects in the outside world can be signalled by changes in many different kinds of motion cues, such as changes in colour, texture and light or luminance levels, to name but a few. We wish to examine how motion that is defined by various kinds of information is handled by the visual system and to what extent these cues are combined in the overall analysis of stimulus speed. In behavioural experiments we will measure the ability of human subjects to detect differences in speed between pairs of moving stimuli. In particular, we will examine how these speed judgements can be affected when the stimuli are defined by differing motion cues and how judgements can be affected by the presence of additional moving stimuli which may be of a similar or dissimilar nature to those stimuli being directly compared. How performance is affected under these conditions will allow us to gauge to what extent various stimulus attributes are used by the visual system in the analysis of stimulus speed. 2) Where in the human brain does the analysis of speed take place? Recent experiments on monkeys have identified areas of their brains that contain neurons which are sensitive to the speed of moving objects. By comparison, relatively little is known about whether equivalent regions of the human brain perform a similar role in the analysis of stimulus speed. We want to identify which regions of the human brain are important in the analysis of object speed. To address this second question, similar behavioural experiments will be used in conjunction with a technique known as transcranial magnetic stimulation (TMS). This technique allows us, without harming human subjects, to momentarily disrupt the operation of small regions of the brain with magnetic fields. Basically, if certain areas are crucial for the analysis of stimulus speed, then we would expect that disruption to those areas would lead to major reductions in performance when subjects are asked to make judgements about the speed of visually presented stimuli. By stimulating different areas of the human brain we can assess their relative contributions to the perception of stimulus speed. The combination of behavioural measures of visual performance with TMS will enable firm conclusions to be drawn about the organisation, timing and location of the neural processes that underpin the perception of speed in the normal human visual system.

This research addresses two basic questions concerning how the human visual system analyses the speed of moving visual stimuli. The first focuses on how the visual system analyses the speed of objects, the motion of which is defined by different kinds of cues. Predictions based on parallel processing models suggest that analysis of the speed of moving chromatic and luminance stimuli occurs within different processing pathways within the visual system. This segregated view is supported by some behavioural experiments, whilst others suggest that information is used in a more integrated fashion. In order to assess how the visual system utilises colour and luminance cues, speed matching experiments will ascertain to what extent the composition of moving stimulus can affect the ability of observers to make speed discriminations. This approach is linked to knowledge that the perception of speed is not always veridical. Under certain circumstances speed can be misperceived, so as a measure of segregation or integration between different kinds of motion cue, we wish to explore how colour and luminance composition can influence the magnitude of these misperceptions. The second question concerns the visual areas in the human brain that are important in the perception of speed. Work on monkeys has highlighted a prominent role for area V5/MT. We wish to define whether V5 plays a similarly central role in speed perception in the human brain. V5, and other visual areas, will first be identified and localised using magnetic resonance brain imaging. Transcranial magnetic stimulation (TMS) will then be used to generate transient neural disruption in these areas. Used in conjunction with behavioural experiments this will allow assessment of the relative involvement of different visual areas, particularly area V5, in different aspects of speed perception such as speed discrimination, the integration of colour and luminance information and short-term visual memory for speed.