Oct24

Oct 24
Dark adaptation

Because rods are relatively insensitive to red wavelengths, being exposed to light in that part of the spectrum doesn't affect those photoreceptors much (ie don't deplete their photopigments)

This means that someone working under red light can effectively keep their eye dark-adapted, so if they need to quickly do something important in the dark, they don't have to wait for their eyes to adapt

Young-helmoltz trichromatic theory: emerged from observations that any colour can be created by mixing together varying quantities of blue, green, and red This theory proposed that there are three different types of colour receptors in the retina (three types of cones), corresponding to each of these three colours

Additive colour theory: applies to mixing lights, colour perception of light is determined by what wavelengths are present or added.

Subtractive colour theory: is different and applies to mixing paints colour perception of paints are determined by what wavelengths are absorbed or subtracted out (blue paint looks blue because it absorbs all wavelength expect for what we idenity as blue.)

While all cones can be stimulated by most wavelength between 400-700nm, each one is maximally sensitive (will produce the largest respoese) to wavelengths corresponding to what we percieve as bue, green, or red.

It was hypothesized that the visual system could combine activity from these cells to encode colour via an additive mixture of impulse.

When theories cannot explain something, this suggests they are incomplete and/or at least some detials are wrong

The trichromatic theory: cannot explain the fact that people with red-green colour blindness should not be able to see yellow according to this theory because red+green= yellowyet they can

Afterimages: an image in a different colour appears after a colour stimulus has been viewed for a prolonged period of time and then withdraw.

If you stare at something green for long enough before looking away, this can lead to the perception of red

Negative colour images demonstrate opponent processes occurring somewhere in the visual system.

Opponent-process theory: in contrast to the trichromatic theory, this theory proposed that we have three cone types, each of which corresponds to two different wavelengths, one is blue or yellow, one is green or red, one is white or black.

This kind of opponent process can explain afterimages, because neural processes supporting those colours get adapted/fatigued.

When you look away while looking at a white surface which stimulates all the cones, there is a rebounc effect and your system interprets the relative difference in activity across the cones

Dual-process theory: combines elements of trichromatic and opponent proves theories, we have three cone types that are each maximally sensitive to three different wavelengths and opponent processes occur in gangliion cells and part of visual cortex

Trichromats: have normal colour vision with systems that are sensitive to red-green, yellow-blue, and black-white

Dichromats: have deficient in one cone system, most commonly red/green but can also be blue/yellow

Monochromats: are sensitive to black/white only

Screens on most electronics emit light in the blue spectrum which reduces melatonin production

This relationship likely has evolutionary origins (blue sky early in day and red at sunset)

While that activity that takes place in the retina is critical for visual transduction to occur, this is the only beginning of visual processing

Neural impulses flow from eye to thalamus to primary visual cortex in the occipital lobe

Feature detectors are neurons that are turned on to respond most strongly to specific stimuli

Many relatively complex shapes can be built from a handful pf basic shapes, meaning these feature detectors make up the basic builiding blocks of many visual representations

Reitinotopy: two points that are close together on an image formed in the retina will also be processed in areas of cortex that are close together In other words, there is a reliable relationship between where simulation on the retina occurs and what part of the visual cortex processes signals related to stimulation at that retinal location

Somatopy: in the somatic sensory cortex

The fovea has enlarged representation in the visual cortex

Different modules in the brain simultaineously process information related to properties like colour, movemnt, shape, distance

All of this information gets integrated and passed along to visual association cortex,and where it is interpreted for meaning based on things like prior experience, memories, knowledge

Some more specialized modules respond to more complex general concepts

Researchers have also sometimes found individual neurons that seem to respond to more specific representations though the interpretation of such findings is complicated

The stimulus for hearing are pressure waves or sound waves which represent a form of mechanical energy

These waves are created through expansion and compression of molecules in some conducting medium (water, air, etc) which we then experience as sound

We can distinguish between the physical properties of a stimulus in the environment and the perpetual experience they create in an observer Frequency and amplitude constitutes two fundamental properties of sound waves

Frequency: refers to the number of cycles per second, related to the perceptual experience of pitch, humans can perceive sound in the range of 20-20000 Hz at their peak approximately 20-12000 for older individuals losing their hearing

Amplitude: quantifies the amount of compression and expansion of molecules in conducting medium, related to the perceptual experience of perceived loudness, measured in decibels

Decibels absolute threshold designated at 0 and this values in measured on a logarithmic scale