Oct21

Oct 21

Psychophysics refers to study of the relationship between the physical characteristics of stimuli and sensory experience

What are the absolute limits of sensitivity? how bright does a light have to be before we see it? refers to a concept called the absolute threshold

What is the smallest difference we can detect? how much does a light have to increase in brightness before we notice it's gotten brighter? related to a concept called difference threshold.

Absolute threshold:Intensity at which stimulus can be detected 50% of the time

Threshold and sensitivity have an inverse relationship- the lower the threshold the higher the sensitivity.

As research in psychophysics continued, it became clear that people don't always have a fixed absolute threshold but rather make decisions that are influenced by various factors.

Signal detection theory: addresses the question of what factors influence sensory judgments

Decision criteria: refers to the standard of how certain we must be before deciding whether or not we believe a stimulus is present. This criteria fluctuates based on various factors such as fatigue, expectations, importance of stimulus, etc.

Standard SDT experiment: series of trials asking is stimulus is present, varying presence/intensity. 4 possible outcomes: hit, miss, false alarm, correct rejection.

take the table from the graph and add to your kownt.

Someone demonstrating very conservative criteria might have fewer false alarms but also miss more hits.

Present and perceived: hit.

Present and not perceived: miss.

Absent and perceived: false alarm.

Absent and not perceived: correct rejection.

Difference threshold: can be called just noticeable difference or JND, the smallest difference between two stimuli that can be detected 50% of the time.

Weber's law: difference threshold is directly proportional to the magnitude of the stimulus with which the comparison is being made, which can be expressed as a Weber fraction

Weber fraction: can differ depending on the modality, eg this fraction for weight is about 1/50. starting with a 50 gram weight, a second comparison weight will be detected as heavier if it's at least 1 gram heavier. Starting with a 500 gram weight, a second comparison weight will be detected as heavier is it's at least 10 gras heavier.

The relationships conveyed by weber fractions tend to hold at normal intensity levels but can break down at extremely high and low intensities.

These fractions can differ across modalities (vision vs smell), properties (pitch vs loudness), species, etc.

Sensory neurons in all modalities respond to constant level of stimulation by decreasing their activity This is adaptive and frees senses to be more sensitive to changes in environment

Sensory adaptation refers to this diminishing sensitivity to an unchanging stimulus (sometimes also refered to as habituation)

Light: the stimulus for vision

Stimulus for vision is electromagnetic energy, or light waves (measured in nanometers or billionths of a meter)

Humans can percieve a small part of the lectromagnetic (EM) energy spectrum from 400 nm to 700 nm

Other species sensitive to other parts of this spectrum

Lens: elastic structure that changes form to help focus, becomes thinner to focus on more distant objects and thicker to focus on nearby objects

Retina: multi layered structure at the back of the eyes that contains light-sensitive photoreceptors which transduce light energy into electrical impulses.

Cornea: transparent protective structure at front of eye

Pupil: adjustable opening behind cornea that controls amount of light entering the eye, the pupil dilates/constricts to allow more/less light into the eye depending on lighting conditions in the environment.

Cameras work very similarily to the eye and can be a useful point of comparison (particularily if you already know the parts of a camera)

The ability to see clearly depends on being able to accurately focus an image directly onto your retina.

Myopia: nearsightedness, difficulty seeing far away objects, light focused too far in front of retina

Hyperopia: farsightedness, difficulty seeing close up objects, light focused too far behind retina

Rods: a photoreceptor, contributes to black/white vision, functions best in low illumination, 500 times more sensitive to light than cones, found throughout the eye but not in the fovea, increasing in concentration towards periphery of retina

Fovea: center of retina

Cones: photoreceptor, contributes to perception of colour and detail, functions best in high illumination, fovea contains only cines, less numberous than rods

Bipolar cells: recieve signal from the rods and cones, more rods are typically connected to a given bipolar cells as compared to cone which contributes to their greater sensitivity to light on account of being able to funnel more info/signal through

Ganglion cell: synapse with the bipolar cells, 1 million in total, the axons of these cells form a bundle called theoptice nerve

Rods/cones to bipolar cells to ganglion cells to optic nerve

Horizontal and amacrine cells modulate signals in the pathway described in various ways

Photopigments

Reaction of photopigments (protein molecules in retina) is critical for visual transduction, which undergo a chemical reaction called isomerization to transduce light into neural code.

Absorption rate of light changes rate of neurotransmitter release from photoreceptors

Greater rate of release = stronger signal passed on

This chemical reaction temporarily deactivates them, which means they can't be used again to transduce light until enough time has passed for them to regenerate.

The bundle of axons that form the optic nerve creates a blind spot where it exits the eye because there are no photorecepors ther.

The human visual system is adapted for both low and high light conditions. This is in contrast to other animals, which may only have rods or cones.

Dark adaption refers to the progressive improvemnts in brihgtness sensitivity in low lighting conditions

This process revolves around changes in the proportion of availabke (ready to transduce light) to used (recently isomerized, and thus in the process of regenerating) photopigments

Cones adapt after 10 minutes (reaching their max sensitivity)

Rods adapt after 30 minutes

Can then detect light intensities 1/10000 as great as before adaptation.

Pigment bleaching accounts for being temporarily blinded by a bright light because your entire supply of pigments is depleted and must regenerate before transduction can occur again.

Along similar lines, the gradual movement that we experience as we dark adapt has a lot to do with pigments being regenerated at a faster rate than they're being used.