The ‘when’ pathway of the right parietal lobe L. Battelli A. Pascual - LeoneP. Cavanagh

What, where and when A central concept in visual research is how we identify objects ( what ) and how we locate those objects ( where ). But an equally important ability is how we compute when visual events occur. In recent years, there is growing interest in understanding the psychological and neuronal bases of the temporal dimension in normally functioning brain and in neurological population.

But time is infinite The relative timing of events underlies an enormous range of neural functions, from microsecond delays of auditory processing to the measure of the seasons. Some suggest a common neural mechanism for all timing operations. Others claim timing is distributed among different structures. The focus here is on the intermediate level, across intervals of up to one second in duration. The class is ordinal - the judgment of order of events in a series, a judgment that may support motion perception. This scale judgment is considered cardinal to many cognitive functions. It is proposed that at this scale, ordinal time is computed centrally.

Where to look for ordinal time in the brain? Evidence that relate time and space computation to the right inferior parietal lobe (IPL) A collision with attention Husain et al. 1997

Where to look for ordinal time in the brain? Other psychophysical and neurophysiological data suggest that the right IPL is related to computing the order of events. The claim is the auditory modality is shares the visual temporal processing mechanism in the right IPL.

Ordinal time or event order and high order motion Motion perception is critically dependent on the discrimination of event order in time and space, and psychophysical studies have shown at least two motion systems. A low level system that computes motion the direction selectivity of neurons in the primary visual cortex.(Hubel and Wiesel). This system can no longer contribute to motion perception when motion must be perceived between a set of discrete stimuli flashed in a sequence, separated by space and time intervals, or when motion is revealed by attentive tracking- a high level attention based motion mechanism, Which is anatomically distinct from low level motion system.

Ordinal time or event order and high order motion- the same? The authors propose that the underlying mechanism for high level motion is the when pathway, the disruption of which can cause dramatic bilateral damage to high level motion perception and, more generally, event order discrimination. Supporting evidence for this claim comes from a neuropsychological study and a corroborating imaging study Battelli et al 2001Claeys et al. 2003

The right parietal lobe and control of transient attention Our perception of objects around us is not merely an accurate registration of their physical attributes. Instead, we shape the world around us into meaningful grouping. Apparent motion is a typical example- what is presented to the retina is not what is perceived. Recent studies show that attentional mechanisms are involved in this phenomena- seen earlier as a high order motion system. Both spatial and temporal properties determine the perception of apparent motion and this enables to ask whether spatial and timing are the same neural mechanism or different functions in the brain. As mentioned earlier the right parietal is the usual suspect. A temporal effect in both visual fields as compared to the known spatial contralateral effect will provide an answer.

Discriminating order of visual events Lights moving or flashing. The slow rate needed for discrimination is due to the attentional load. Right parietal patients Show a deficit in both Visual fields- when Pathway. Deficits are due to Accuracy in discriminating Offset and onset. Neglect patients have no deficits with motion perception.

Determining the order of visual events To determine if the apparent motion was due to offset onset confusion. An asynchrony Flicker task. Patients did well as controls when offset and onset were not overlapping In time. Transient object events like appearance and disappearance require temporal attention. Thus the results for a bi lateral temporal component.

Interpreting spatiotemporal information if visual events Corroborating evidence from right IPL TMS studies that showed a contra lateral spatial effect and a bilateral temporal component. Biological motion is another visual stimuli that require spatiotemporal attention- an apparent motion paradigm. Parietal patients and TMS on right IPL interfered with biological perception- though the spatial and temporal components were not dissociated. Left parietal patients showed interference with perception of a single dot biological motion. Indicating the left parietal in action observation and movement control.

Concluding remarks Recent studies(husain and rorden 2003) show the right IPL is involved in non spatial visual processing. Experimental evidence reviewed show a role of right IPL in control of attention over time. Here the when pathway is proposed to accommodate these new findings. This attentional mechanism is important in discrimination of two visual events that occur on the same space but different time intervals. This may explain heterogeneous deficits in visual neglect patients. Since the temporal deficit is bi lateral it may confound ‘’spatial’’ explanations of neglect. Also it suggest additional therapeutic aims for the patients.

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