Proceedings of The Physiological Society

University College London 2006 (2006) Proc Physiol Soc 3, PC215

Poster Communications

Feedback effects from cortical area MT modulate response properties of lateral geniculate nucleus cells

Helen E Jones1, Wei Wang1, Ian M Andolina1, Thomas E Salt1, Kenneth L Grieve2, Javier Cudeiro3, Adam M Sillito1

1. Institute of Ophthalmology, Dept of Visual Science, UCL, London, United Kingdom. 2. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom. 3. Dept. Med-Neurocom, University of A Coruna, Coruna, Spain.

  • Figure 1. Top location of MT and LGN cell receptive fields. Middle iconic representation of stimuli. Bottom LGN cell responses before (Con) during (GABA in MT) and after (Rec) focal decrease of MT visual responses to a grating drifting in the preferred (left) and non-preferred (right) directions of the MT cell.

The visual cortical motion area, MT/V5, provides a feedback projection to layers 1, 4B and 6 of the primary visual cortex (V1). It has been recognized for some time that the projection to layer 6 of V1 provides the potential for MT to influence the cells providing corticofugal feedback to both the magno and parvocellular streams (Shipp & Zeki, 1989). Here we report data showing that very focal pharmacological manipulation of response magnitude in area MT of the macaque, by iontophoretic application of the GABAB receptor antagonist CGP 55845, produces clear changes in the response properties of cells in the lateral geniculate nucleus (LGN) in magno, parvo and koniocellular streams. Animals were premedicated with atropine sulphate (0.04mg/kg i.m.) and acepromazine maleate (0.05mg/kg i.m.). Anaesthesia was induced by injection of ketamine (10-15mg/kg i.m.). Surgical procedures were carried out under ketamine anaesthesia (10-15mg/kg i.m.). Bupivicaine hydrochloride (0.75% w/v) was applied to all wound margins. Throughout the course of the experiment, anaesthesia was maintained with sufentanil (4-8μg/kg/h i.v.) and a mixture of 70% N2O and 30% O2. Recordings were carried out in the presence of neuromuscular blockade (0.1mg/kg/h vecuronium bromide i.v.). See Jones et al. (2001) for full details of the precautions taken to ensure adequacy of anaesthesia. We took data from 55 LGN cells (30 parvocellular, 15 koniocellular and 10 magnocellular layer cells). In the presence of enhanced visual responses in MT (monitored by simultaneous recording at the site of drug application) we observed significant and reversible changes in visual response magnitude for 75% of these (41/55 cells). Changes in response magnitude were seen in cells in parvo-, konio- and magno-cellular layers (23 parvo, 10 konio and 8 magno). Of these, approximately half (21) showed response increases (mean increase 116±33% (SEM)) and half response decreases (mean decrease 40±5% (SEM)). Increases and decreases were seen in magno, parvo and koniocellular cells. In a further experiment we compared the responses of LGN cells to a grating drifting in the preferred direction of cells at the MT drug application site and in the opposite direction. In the presence of drug application (in this case iontophoretic application of GABA to reduce MT responsiveness) we observed directionally specific effects (see Fig. 1) on 4/8 of the LGN cells tested. These preliminary findings suggest that feedback from MT via V1 influences the responses of LGN cells to moving stimuli and has the capacity to modulate their responses in a directionally specific manner.

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