Sensory Modalities

Question 1

BRIGHT LIGHT/PHOTOPIC SYSTEM

Answer 1

• receptor = cones
• approx receptors p/eye = 4m
• photopigments = 3 cone opsin classes; colour vision basis
• sensitivity = low; needs strong stimulation; used for day vision
• retina location = concentrated in/near fovea; present less densely throughout retina
• receptive field size/visual acuity = small in fovea; high acuity; larger outside fovea
• temporal responses = relatively rapid

Question 2

DIM LIGHT/SCOTOPIC SYSTEM

Answer 2

• receptor = rods
• approx receptors p/eye = 100m
• photopigments = rhodopsin
• sensitivity = high; stimulated by weak light intensity; used for night vision
• retina location = outside fovea
• receptive field size/visual acuity = larger; lower acuity
• temporal responses = slow

Question 3

DICHROMATIC COLOUR VISION

Answer 3

• most mammals/dichromats have it; lack M-cone
• trichromats (humans/old-world primates) discriminate more colours than dichromats
• marine mammals (whales/seals) don’t see colour (only L-cone)
• S-cones = short-wavelength sensitive receptor
• M-cones = medium-wavelength sensitive receptor
• L-cones = long-wavelength sensitive receptor

Question 4

COLOUR VISION DEFICIENCIES

Answer 4

MELILLO et al (2017)

• mild colour deficiency = trichromats
• colour blindness/severe deficiency = dichromats; truly colour blind; men > women affected (X chromosome defects are frequent; men only have X chromosome)

Question 5

HEARING

Answer 5

• to detect/discriminate locations/movement of sound sources
• spatial orientation
• echolocation (bats/whales/humans)
• auditory communication/language

Question 6

SOUND

Answer 6

• pressure waves; air particle movement set in motion by vibrating structure
• propagates in 3 dimensions alternating compression/rarefaction of air molecules move back/forth from regions of high/low pressure
• measures of sound:
  1. frequency (reciprocal of wavelength; perceived as pitch)
  2. amplitude (loudness)/phase/waveform

Question 7

SOUND WAVES MIX/IMPINGE ON TYMPANUM

Answer 7

• dif to retina; ear cannot spatially map sound locations

- vibrations travel from tympanum to middle ear where they’re amplified

Question 8

TONOTOPIC HAIR CELL ARRANGEMENT

Answer 8

• mapping by sound frequency

- hair cells tuned to narrow sound frequency range by their location along basilar membrane

Question 9

STEREOCILIA (STIFF HAIR)

Answer 9

• help stretch open ion channels
• stereocilia bending (input zone)
• nonselective ion channels opening allow K+/Ca2+ ion influx
• neurotransmitter released to excite afferent nerves
• hair cell depolarisation opens voltage-gated Ca2+ channels in base of hair cell (output zone)

Question 10

AUDITORY NERVE

Answer 10

• spiking auditory interneuron axons
• IHC needed for hearing
• OHC can change length to fine tune Corti organ
• hair cells innervated by first-order interneurons (afferent/efferent)

Question 11

AUDITORY TUNING CURVES

Answer 11

AUDITORY INTERNEURONS
- electrophysiological recordings from 6 cells in cat’s auditory nerve
PERCEPTION/BEHAVIOUR
- psychophysical experiments measuring hearing threshold
- comparisons between species show dif sensitivity ranges

Question 12

AUDITORY PATH: RECEPTOR -> PRIMARY CORTEX

Answer 12

• most projections from cochlear project to contralateral cortex
• each superior olivary nuclei of brainstem receives inputs from both cochlear nuclei for first binaural analysis stage of sound-source location
• inferior colliculi located in dorsal midbrain
• medial geniculate nuclei of thalamus

Question 13

SUMMARY I

Answer 13

• colour vision (only bright light) depends on 3 cone types in retina which express either S/M/L opsin sensitive to wavelength range
• colour-sensitive neurons in P-pathway/ventral stream (ganglion cells/LGN/V1/2/4) generate conscious colour percepts/mediate colour constancy to adjust for changes in spectral composition of sunlight through dawn/day/dusk
• animals have no colour vision: dichromatic (mammals)/trichromatic/tetrachromatic colour vision
• humans have normally trichromatic colour vision but sometimes one opsin = missing/shifted in spectral sensitivity causing severe -> mild colour deficiencies respectively; deficiencies arise from defective opsin gene copies, most frequently in X chromosome (coding for M/L opsin) so colour deficiencies more common in males

Question 14

SUMMARY II

Answer 14

• light/sound propagate as waves that differ in frequency/intensity
• to locate sound sources animals compare sound info processed by both ears
• while light absorbed by photoreceptor as quanta, sound vibrates internal ear structures
• hair cells have stereocilia/ion channels open via mechanical forces; inner hair cells release glutamate to excite afferent first-order auditory interneurons that transmit signal to cochlear nucleus/brainstem
• signal coding modulated by outer hair cells/efferent interneurons (top-down control)
• auditory path has parallel/serial connections similar to vision
• audiograms allow comparisons between species to determine how hearing can be adapted to dis tasks/ecological needs