Area V1 has a retinotopic map (and so do other visual areas) f o v e a left visual cortex right visual field 90 45 inferior vertical meridian 40 270 f o v e a 20 315 2.5 5 10 20 40 10 HM 0 = HM 10 45 90 superior vertical meridian 1 cm 315 270

retinotopic mapping using fMRI (usefull for area localisation)

A B C CHARTING VISUAL AREAS WITH fMRI Determine separate visual areas by charting visual field maps. These can be rendered (A) on brain surface image; (B) on brain ‘balloons’ (inflate volume to flatten out sulci, shown in dark grey; (C) on totally flattened 2D surface.

Ηλεκτροφυσιολογία (Electrophysiology) η έννοια του representation (mental vs. neural) ηλεκτρόδια στον εγκέφαλο anaesthetised vs awake behaving αντιδράσεις νευρώνων σε ερέθισμα (recording/καταγραφή) single vs. multi unit recordings (SUA vs MUA) multiple electrode (cell) recording (πολλά ηλεκτρόδια) receptive field of a neuron συχνότητα εκφορτίσεων (rate coding, spikes/s) ταλαντώσεις, συγχρονισμός εκφορτίσεων (oscillations – correlation) alternative codes: single spike timing, spike trains etc. single neurons as feature detectors (orientation, colour, direction of motion etc.) i.e. the single neuron as a filter allowing selective passage of information grandmother cell vs. population coding microstimulation

Extracellular Recordings Alan Peters (1994), Cerebral Cortex Alan Peters Cerebral Cortex, Vol 10, 1994 100 mm Whatever the general aim may be in such experiments: 1. Animals are typically trained to perform a visual task; that is report on the properties of a visual stimulus by making EM or pressing levers… 2. And we record by means of microelectrodes, as shown here Measured is the so called mean extracellular field potential. That is the linear sum of all sinks and sources generated by active and inactive membrane-regions respectively. The superposition principle valid for a quasi-static ohmic volume conductor predicts that such measurements can be only done when the geometry of the cells embedded in the conductor permits it. Cortex, but also cerebellum and hippocampus are optimal structures:

spike raster & PSTH

spike-trains from 30 neurons

Hubel & Wiesel: orientation selectivity in V1 For example, cells in V1 fire when a line of the appropriate orientation is place within their receptive field This is in the very beginning of the visual pathways, which you can see in the next slide

receptive fields in V1 act as both orientation and spatial-frequency filters

Hubel & Wiesel 1970-1980

METABOLIC COMPARTMENTS IN V1 & V2: blobs & stripes brain slice in plane of layer 3; cytochrome oxidase histology

εμπειρία + περισσότερα κύτταρα στη V1 προτιμούν κάθετο/οριζόντιο προσανατολισμό (αλλά άτομα με αστιγματισμό το ανάποδο, ακόμα και μετά τη διόρθωση!) V1 orientation selectivity in retinal rather than in world coordinates (monkey experiments) perceptual effect also tied to retinal coordinates (tilt your head, V1 origin)

feature detector-grandmother cell vs. population coding monocular vs feature detector-grandmother cell vs. population coding monocular vs. binocular mechanisms (i.e. colour induction) adaptation-fatigue-aftereffect-binocular transfer (i.e.MAE)