2009 Conferences		Contents

Abstracts

Gaze polling and fixation shifting of cyclists negotiating a slalom

Wilkie R.M. & Wann J.P

Following on from our work investigating gaze and steering along simulated roadways (Wilkie & Wann, 2003; 2004) we examined gaze behaviour in a steering task that allowed for a greater degree of route selection. We integrated a bicycle with our simulation rig that allowed more precise measurement of the paths taken, due to narrowness of the bicycle wheel. Participants steered around a series of obstacles in a simulated slalom of varying complexity. We recorded patterns of gaze sampling alongside the mean passing distance from obstacles, the overall variability of path, and the smoothness of each trajectory. Gaze responses were categorized as either gaze 'fixations' on the most proximal obstacle (O_n), gaze 'polling' of more distal obstacles (a saccade to O_n+1 and then refixation of O_n) or gaze 'shifting' (saccade and fixation of O_n+1 with no refixation of O_n). Increased complexity of the course caused more steering errors, with greater path variability and reduced steering smoothness. While the mean timing of gaze shifts did not change, they did exhibit relatively large variability across both obstacles and participants reflecting the changing demands of the task and the skill of the participant. Gaze polling was only intermittently observed, except in one participant who was particularly well practiced. She also displayed the greatest accuracy and smoothness in steering, suggesting that gaze polling is only employed at an advanced state of locomotor control. We discuss the relationship between gaze polling, fixation shifting and steering performance in relation to our current steering model (Wann & Wilkie, 2004).

Research supported by the UK EPSRC GR/S86358.

Perceiving time to collision activates sensorimotor cortex

Field D.T. & Wann J.P

The survival of many animals hinges upon their ability to avoid collisions with other animals or objects, or to precisely control the timing of collisions. Judgements of time-to-collision (TTC) can be made using optical expansion information (Lee, 1976) and this may be combined with binocular information (Rushton & Wann, 1999) .  Neural systems underlying TTC judgement from optic expansion have been identified in pigeons (Wang & Frost, 1992) and houseflies (Wagner, 1982) . In the case of humans, what cortical areas process judgments of TTC from optical expansion or how these are linked to co-ordinated action is unknown. We investigated this question using fMRI. Observers judged which of two approaching objects would strike them first (Todd, 1981) , using optical expansion information. The main control task involved judgements of objects that inflated while remaining at a fixed perceived depth. A second control task utilised TTC judgements for objects translating in the frontoparallel plane. We identified areas of superior parietal and motor cortex, which are selectively active during perceptual judgments of TTC with the point of observation, some of which are normally involved in producing reach-to-grasp responses.  These activations could not be attributed to actual movement of participants. We demonstrate that networks involved in the computational problem of extracting TTC from expansion information have close correspondence with the sensorimotor systems that would be involved in preparing a timed motor response, such as catching a ball.

Research supported by the UK EPSRC 

How Would You Catch A Ball If You Had Visual Form Agnosia?

Wann, J.P., Field, D., Mon-Williams, M., Milner, A.D.

Visual form agnosia offers a means for exploring the capabilities and, to some degree, the limitations of the human dorsal visual stream. Wann et al (2001) observed that a patient with visual form agnosia (DF) could modify appropriately her reach towards an object if there was a rapid change in its disparity-specified distance but in contrast to control participants a sudden change of optical size did not produce an equivalent modification in reach distance. This raises the question of how sensitive is DF to looming information that ordinarily signals object approach and time to collision (TTC)? We first established that DF has a reasonable ability to catch a ball that is thrown to her in a straightforward fashion. We next presented DF with a range of looming and changing size stimuli. DF was able to report verbally whether an object was approaching or receding when shown a simulation of a looming or contracting ball but was unable to make the same judgment when presented with an equivalent step-change in the size of static images (a task that appeared trivially easy to controls). When asked to hit a button when she gauged a looming ball would hit her, she graded her responses with changing TTC but seemed to rely upon optic size rather than the relative rate of dilation (Tau: Lee, 1976). We tested control participants (female, 48-56yrs) and found that they also failed to grade their responses in line with Tau. We will discuss how the skilled performer might extract a perceptual estimate equivalent to Tau from changing size, how this might degrade if high precision tasks are not practised and why this mechanism might fail in the case of visual form agnosia.

This research was supported by the UK EPSRC. The authors are extremely grateful for the wonderful cooperation provided by DF.