Cells Sensitive To Orientations And Spatial Frequencies example essay topic

How have adaptation experiments contributed to the understanding of visual perception? Perception involves the input of basic information from a retinal image. Retinal images can be ambiguous in terms of size and orientation of lines, blood vessels infront of the retina, eye tremors and the blind spot can affect perceptual responses. One retinal pattern can lead to different interpretations and distorted images from inadequate sensory data, but perception of something as a recognisable object remains stable despite variations in the retinal image, such as light, or position of the object. Perception is a process of construction, a stable, veridical or true representation of the world which needs to be constructed. The retinal image can be ambiguous due to different sizes, slants or distances, but still leads to a particular interpretation of the object.

Eye movements are not a series of fixations, clips of static images, but smooth movements resulting in a continually moving image. Shadows of blood vessels over receptor cells in the retina can lead to distortions in the retinal image, but the eye takes information from the environment to the brain for a perception to be constructed. People do not see 'snapshots' of the world, the eye and optic nerve does not send pictures to the brain, it transmits information regarding the patterns of light that hit the retina. The visual pathway involves the retina, which includes photoreceptors, rods and cones.

Rods are sensitive to dark and light, cones are responsive to colour. Both form a layer at the back of the retina. Retinal ganglion cells (RGC) form another layer infront of the photoreceptors, RGC axons leave the eye and form the optic nerve. Adaptation occurs in the retina to adjust the sensitivity of receptors and vary the light intensity. Sensitivity is mainly adjusted by adaptation to match the light intensity at that moment. There is also light adaptation, going from a dark to bright environment, a quick process, and dark adaptation going from a bright to dark environment which takes longer.

Items can be recognised by a key or sign stimulus of the object such as angels or features that don't vary even if the image changes. Barlow (1972) suggested that single cells in the visual pathway act as feature detectors, cells are tuned to respond when particular features are present. Barlow also proposed that cells and neurons are arranged in a hierarchy with high levels being tuned to more aspect features. Hubel and Wiesel (1968) also proposed a hierarchal system with retinal ganglion cells on lower levels detecting light and dark edges at any positions or orientations. Simple cells formed a second layer, tuned to more specific features such as certain orientations.

A third level contained complex cells that also detected edges with particular orientations. Hyper complex cells detect particular orientations and lengths, the cells are feature detectors and lead to object recognition in perception. Adaptation effects have been seen in experiments with gratings, patterns of alternating dark and light bars such as black and white bars, looking like a bar code. Each column of black and white are the same size but gratings may be of different spatial frequencies or widths.

Frequency is measured as cycles per degree, per unit of visual angle. A degree is the amount of retina taken up by the image viewed by a distance of 57 cm for an object 1 cm high. One cycle refers to one black plus one white bar that takes up a certain width, the narrower the bars the higher the frequency and finer the detail. Gratings can vary according to their luminance profiles, that is the degree of contrast between the colours of the bars in a cycle such as in square and sine wave distribution gratings. Square wave gratings consist of light and dark bars of the same contrast, each black bar is the same degree of blackness all the way through the bar, the same for white columns. There is a high degree of contrast where black finishes and white begins.

Sine wave bars have a stronger colour in the middle and lighter towards the outside so that the end of one bar merges with the beginning of the next bar. Contrast threshold can be measured for certain spatial frequencies in the minimum contrast ratio perceived between dark and light bars. The contrast is the difference between intensities of light in a cycle. It is possible to find a person's contrast level which is just detectable on a grating of any spatial frequency. After staring at a particular spatial frequency a person can adapt to it, there is an elevation in contrast threshold and decrease in sensitivity.

If the same grating was stared at again after adaptation from the first time, the person would not be able to detect a contrast between bars and cycles as well as the first time. This shows that receptors of cells 'tune in' to particular spatial frequencies and respond strongly to them, the response being measured by contrast sensitivity, En roth-Cu gell & Robson (1966) cited in Bruce and Green (1990). This is one feature that aids object recognition and perception of items. When adaptation to a grating takes place certain responses are measured.

Adapting or inhibiting certain cells that are tuned in to particular aspects, such as contrast, will show up in a further test after the adaptation has taken place. This can be seen as an altered perception, or detection threshold in gratings. In looking at a grating for an amount of time, contrast threshold is temporarily elevated for detecting another grating similar in orientation and spatial frequency. The threshold elevation can also be used to investigate orientation sensitive channels in the visual system, contrast is orientation specific for the frequency that has been adapted to. In an adaptation experiment to sine wave gratings for 60 seconds, orientation of the grating was varied between 0 and 40 degrees, contrast remained the same. Orientations between 10 and 20 degrees of the original position had an effect shown in an elevated contrast threshold.

(Beaton and Blakemore 1981) Simple cells have an orientation preference showing large responses to edges at particular angles. The preference is narrow and turning the angle of an edge more than 20 degrees will reduce the firing rate of the cell. Studies of single cells in the primary visual cortex of monkeys and cats have revealed cells sensitive to orientations and spatial frequencies, suggesting that human visual systems are organised in a similar way. Micro electrodes have been used to examine behaviour and firing rate of cells in a cat's cortex. Strips of light were shown on a dark background, a typical cell would respond best to their preferred orientation but also 10 to 20 degrees to each side of their optimum. (Hubel and Wiesel 1978) Orientation and spatial frequency specificity of adaptation effects have been used to identify areas in the brain and cells that are tuned into certain orientations, spatial frequencies, movement and depth (retinal disparity).

There is a modularity of visual function, processes occur independently of each other. Channels and cortical pathways process information by organising into separate sections which deal with certain aspects such as form, motion etc. All aspects are then brought back together through perception. A preference for certain features informs the visual system of an input and output such as patterns of light that hit the retina which provide information about the environment. The output is the information about surfaces, objects or events in the environment needed for everyday life and activities.

Bibliography

Beaton, A. & Blakemore, C. (1981) Orientation selectivity of the human visual system as a function of retinal eccentricity and visual hemi field.
Perception. 10,273-282. Pion Limited. Bruce, V. & Green, P, R., (1990).
Visual Perception, Physiology, Psychology and Ecology. Lawrence Erlbaum Associates Ltd. Epstein, W., (1977) Visual Perception.
While & Son Hubel, D, H., (1988).