Vision

Light is electromagnetic energy. One nm = one billionth of a meter The Visible Spectrum

Properties of light hue – determined by wavelength. saturation – relative purity of light. brightness – variation in intensity.

Wavelength and Frequency As wavelength increase, frequency decreases

The Human Eye

In order to see things in greatest detail our eyes are moved so that the object being looked at falls on the fovea. Fovea is a central portion of the retina with the greatest visual acuity.

Photoreceptors rods and cones contain photopigment that provides input to bipolar and horizontal cells. photoreceptors and bipolar cells do not produce action potentials – instead release neurotransmitters to the ganglion cells. ganglion cells connect with the optic nerve.

Blind spot Optic disk – where the optic nerve joins the retina – transmits retinal information to the occipital lobes

Blind spot Close your LEFT eye and move head closer to or further away from the screen until the central red circle disappears – always fixate the CROSS.

Visual Fields

temporalnasal VISUAL FIELD RETINA temporalnasal

Primary geniculostriate visual pathway

Primary Geniculastriate Pathway retina optic nerve optic chiasm optic tract dorsal lateral geniculate nucleus optic radiations striate visual cortex

Lateral Geniculate Nucleus The LGNd has six layers each of which gets independent input from either the left or the right eye but not both. There are two major classes of projections, parvocellular (small) and magnocellular (large) projections (known as the P and M pathways).

Magnocellular Parvocellular Large ganglion cellsSmall ganglion cells Centre/Surround Colour insensitive Colour sensitive Large RFs Small RFs Fast, transient Slow, sustained High contrast sensitivityLow contrast sensitivity

Primary Visual Cortex The LGNd projects to primary visual cortex (striate cortex or area V1) in the occipital lobe. The magno and parvo projections are still somewhat segregated in V1.

Retinotopic map in striate visual cortex

Visual receptive fields receptive fields of retinal ganglion cells correspond to specific regions in space – hence a retinotopic map of the world in the occipital cortex. receptive fields in visual cortex also respond selectively to other stimulus properties (e.g., orientation, brightness).

Centre – surround organization tuning – different types of cells are “tuned” to respond to different aspects of visual information e.g., brightness, location, direction of motion, colour etc…

Coding information at the retina - brightness centre / surround organizationorganization ON, OFF and OFF/ON cells ON/OFF cells project primarily to the superior colliculus (midbrain) the SC is important for directing reflexive saccades

Coding information at the retina - colour

trichomatic sensitivity AND colour opponency red – green blue – yellow on/off surround organization Yellow ON Blue OFF Blue ON Yellow OFF Green ON Red OFF Red ON Green OFF

Coding information at the retina - colour impossibility of seeing a redish green colour!

Adaptation – negative afterimages after staring at the green Canadian flag you see a red one because the “green” component of red/green cells has adapted to the stimulus. some red/green cells are inhibited for a long period. when looking at neutral light (white light) these cells “rebound” due to the absence of inhibition creating the afterimage. Big Spanish Castle can get afterimages for motion – waterfall illusion.waterfall illusion

Striate cortex 6 layers (bands or striations). input from magno and parvocellular information processed at layer IV. disproportionate representation of the fovea (brain would weigh over 30,000 pounds (≈13,600 kg) if the whole visual field had as many neurons dedicated to it as are dedicated to the fovea!!!).

Orientation and movement cells in striate cortex sensitive to specific orientations. simple cells – opponent system. complex cells – no inhibitory surround – direction specific movement detectors (also in MT). cells organized in columns.

Spatial frequency many of the cells in striate cortex are actually tuned to different spatial frequencies. everything you see in the world can be described in terms of spatial frequency. low spatial frequency high spatial frequency

Information not lost at low spatial frequencies Gender and can still be extracted from the low frequency image (right) but identity requires the high frequency image (left).

Modularity in vision Different “modules” sensitive to different visual processes V4 – colour MT – motion FFA – face perception PPA – place recognition IT – object recognition

1) A unique feature of the fovea is that it A) contains mostly rods. B) contains mostly cone photoreceptors. C) is devoid of photoreceptors. D) mediates vision in dim light. E) has very poor acuity. 2) The reason for a "blind spot" in the visual field is that A) rods are less sensitive to light than are cones. B) blood vessels collect together and enter the eye at the blind spot. C) the lens cannot focus all of the visual field onto the retina. D) retinal cells die with age and overuse, resulting in blind spots. E) there are no photoreceptors in the retina where the axons exit the eye. Review Questions

3) Action potentials in the visual system are first observed in the A) bipolar cells. B) horizontal cells. C) ganglion cells. D) photoreceptors. E) axons leaving the internal surface of the retina. 4) Select the correct sequence for processing of information in the primary visual pathway. A) Retina - > dorsal lateral geniculate (DLG) -> striate cortex B) Retina -> striate cortex -> extrastriate cortex -> inferior temporal cortex C) DLG -> retina -> striate cortex -> primary visual cortex D) Retina -> DLG -> inferior temporal cortex -> amygdala E) DLG-> frontal cortex -> amygdala -> extrastriate cortex Recommended web page