Lecture 4: sensation and perception Flashcards
Senses
the physiological capacities to provide input from the environment to our neurological system
Sensation
the translation from information about the environment into patterns of neural activity
perception
our sensory experience of the environment
Although the senses work very differently, they have some anatomical and processing features in common.
Each sensory system:
Starts with an anatomical structure to collect, filter and amplify environmental information
has specializes receptors to transform the environment information in a neural signal that are passed along specific sensory nerve pathways
These nerves end up in the Thalamus and then go to a primary sensory region of the cortex (except olfactory nerve)
Acuity
how good we can differentiate among stimuli within a sensory modality
acuity depends on:
the anatomical structure to collect the stimulus (dogs can move their ears independently to better capture sound waves)
The number and distribution of the receptors. acuity is best in the centre of the visual field because the fovea is packed with photoreceptors.
our sensory systems (5)
olfaction: smell
gustation: taste
Somatosensation: touch
auditon: hearing
vision: sight
Olfaction (smell)
The olfactory epithelium
odor molecules enter the nasal cavity
These molecules bind to bipolar receptors in the nasal mucosa (=The olfactory epithelium)
Olfaction (smell) in the brain
glomeruli & olfactory cortex (pyriform cortex) & orbitofrontal cortex
The axons of the glomeruli from the olfactory nerve that projects to the primary olfactory cortex (pyriform cortex). from there the signal proceeds to the orbitofrontal cortex.
Shape theory (docking theory) of olfaction
Odor molecules bind to some of the thousand different types of receptors because the shape of the molecule corresponds to the shape of the receptor (like a key fits a lock). The odor depends on which molecule binds to which receptor. however some molecules with similar shapes have very different odors
Vibrational theory
Similarly shapes molecules have different molecular vibrations (at quantum level)
olfaction and behavior
Men rated women as less attractive after smelling real tears:
Brain areas related to sexual arousal (hypothalamus & fusiform gyrus) showed a decreased BOLD response while watching emotionally evocative (erotic) clips after smelling real tears.
Gustation (smell) way to the brain
papillae
on the tongue there are papillae and have taste buds.
food molecules (tastants) pass the tongue via the taste pores to the tase buds. each taste bud contains taste receptor cells.
molecules bind to receptor, the axons form the chorda tympani nerve. together with other nerves, this nerve forms the facial nerve VII and projects to the solitary tract nucleus in the brainstem.
from there the signal goes to the thalamus and the primary gustatory cortex in the insula.
Gustotopic map
neurons in the gustatory cortex are organized meaning that their spatial organization follows a property of the environment or body
gustation research in the thalamus showed:
when researchers recorded the thalamus they saw that neurons respond selectively to one taste (sour, sweet, bitter or umami)
somatosensation (touch)
the perception of all mechanical stimuli that effect the body
- the interpretation of signals that indicate the position of the limbs and head (proprioception
- temperature
- pressure
-touch
-pain
-etc..
somatosensation is caused by…
somatosensory receptors under the skin and at the musculoskeletal junctions (where all the muscle fibers meet)
nociceptors
pain is sensed my the nociceptors (free nerve endings)
somatosensation (touch) signal to brain
via the dorsal root ganglion, the signal travels through the spinal cord to the medulla and the contralateral side of the thalamus. the thalamus sends the signal to the primary somatonsensory cortex (the postcentral sulcus)
somatotopic map
neurons in the primary somatosensory cortex are organized in a somatotopic map.
size of the representation is proportional to the sensitivity of the body part (hands are sensitive and have a relatively large representation in the brain)
Audition (hearing
The peripheral auditory system: the external ear & the middle ear
The external ear: collect and focus sound energy
The middle ear: sound energy hits the eardrum (the tympanic membrane) that is connected to the three tiny bones: mallues, incus and stapes.
Stapes are connected to the cochlea via the oval window
The internal ear
Cochlea consists of three chambers, all filles with fluid
the basilar membrane
the basilar membrane inside the cochlea is embedded with hair cells (the tips of the hair cells are called sterocilia)
tonotopic organization
depending on the frequency of the sound, different hair cells will move. the basilar membrane close to the oval window reacts to high frequencies; the membrane at the end of the cochlea (apex) react to low frequencies.
low tones travel far.
The central auditory system
Signal from hair cells is propagated to the brain via the auditory nerve cell that first projects to the cochlear olivary complex. the signals from the left and right ear come together.
then to the inferior colliculus (orientation towards the sound) and medial geniculate complex.
Localizing sounds:
interaural time difference
key idea: differences in length of path to neurons in the medial superior olive (MSO) create delay lines.
speed of sound is slow
neurons in the medial superior olive (MSO) function as coincidence detectors.
Localizing sounds:
interaural intensity difference
head forms an obstacle for high frequencies. a stimulus is presented on the left will cause a higher activity in the left lateral superior olive (LSO). this activity will inhibit the activity coming from the right LSO
only activity from the left LSO is processed in higher centres
Vision (sight)
photoreceptors
rods: primarily concerned with perception at very low levels of light
cones: perception at greater light intentsities. responsable for detail and colour percepts.
rods and cones are not evenly distributed in the retina:
at fovea: mainly cones
away from the fovea: more rods
Vision (sight)
acuity in vision
Acuity: how sharp your vision is (the discrimination ability)
is highest at the fovea and sharply decreases as a function of distance to the fovea.
example: it is easy to count the number of fingers if directly look at them, but impossible if you can only see them in the periphery.
Vision (sight)
Fovea
your eyes five you a clear and color signal for what is present at the fovea. parafoveal: you only have fuzzy and no color information
ipsilaterally
the optic fibers from the temporal half of the retina project ipsilaterally, and the nasal half crosses over the optic chiasm
lateral geniculate nucleus (LGN)
90% of the retinogeniculate pathway projects to the lateral geniculate nucleus (LGN)
1% projects to the pulvinar, both the LGN and the pulvinar are involved in visual attention
Geniculocortical pathway (optic radiation)
the LGN projects to the primary visual cortex V1
primary visual cortex:
retinotopic organization
the spatial layout of the visual field that is projected on the retina is maintained in the thalamus and in V1
objects that are close to each other in the visual field (and hence the retina) are also represented close to each other in the primary sensory cortex.
primary visual cortex:
cortical magnification
the size of each unit area of the retina is disproportionately represented in the visual cortex
visual information at the fovea occupies much larger area in the primary visual cortex compared to the parafoveal information.
receptive field
visual neurons keep track of information about where objects are in space by their receptive fields
Ganglion cells (retina) and LGN cells (thalamus)
respond to light going on and off in their receptive fields
Receptive fields at the cortex
receptive fields at the cortex become more complex because a neuron in V1 receives input from multiple LGN cells. the receptive fields of neurons in V1 are edge detectors.
Jennifer Aniston cells
The further you go in the visual stream the larger/more complex the receptive fields become
Jennifer Aniston cells: It activates when a person “sees, hears, or otherwise sensibly discriminates” a specific entity, such as their
Functional organization of the visual cortex
hierarchical processing
simple cells (edge detectors) in V1 are combined to V2 to form complex cells. information in V2 is integrated in V3 to form hypercomplex cells.
the connections in the visual field cortex do not follow a nice hierarchy
Functional organization of the visual cortex
analytic processing
Each visual area provides a map from the external environment, but each map represents different information (specialized processing)
deficits in visual processing
achromotopsia: a condition characterized by a partial or total absence of color vision. People with complete achromatopsia cannot perceive any colors; they see only black, white, and shades of gray.
akinetopsia: “motion blindness”, which is a higher visual processing disorder from an extra-striate lesion, in which a patient has difficulty specifically perceiving objects in motion with variable severity and rarely complete.
Flicker fusion
the frequency at which flikkering light can be perceived as continuous and is used to assess the processing of temporal vision.
The enigma pattern
where in the brain does the activation correlate with our percept?
the illusion of motion activates the brain area related to motion processing V5!
Multimodal perception
We use all availble information to converge to a coherent represention of the world. we integrate information from different sensory modalities to make sense of the world.
for multimodal integration to occur, the signals from different modalities need to be coincident in time and space
Where in the brain does multimodal perception happen? animal
early studies on hamsters showed that neurons in the superior colliculus are very sensitive to the presentation of multimodal stimuli. (study of visual and auditory)
Where in the brain does multimodal perception happen? human
The superior temporal sulcus (STS) is where different multimodal association has been found. related to the McGurk effect. (ba/ga)
synseasthesia
when senses get mixed. when your brain routes sensory information through multiple unrelated senses, causing you to experience more than one sense simultaneously.
example: tasting words or linking colours to numbers and letters
perceptual reorganization
The somatosensory cortex shows signs of plasticity
after removal of a finger, the brain area that used to represent that finger is now used to represent other nearby fingers.
