Sensation and Perception

Ch 3: Spots to stripes

Dr. Chris Rorden

PSYC450-003

Gutenberg’s Printing Press

• Gutenberg invented printing press in 1440:
• Biggest invention in last 1000 years.
• Ended dark ages, followed by rapid technological revolution.
• Similar technology invented earlier in asia, but did not have profound influence. Why?

Give me a sine

• Sine waves are the building blocks for understanding visual and auditory perception.
• Defined by amplitude and frequency.
• Shape is ‘sinusoidal’ with rounded peaks, unlike other types of waves like those we see breaking on a beach.

The sine wave

• Most oscillations are sinusoidal
• Vibrating guitar string
• Piston in our car engine
• Pendulum swinging left and right
• Dropped mass on spring
• Tidal Height
• Radio Signals
• Sound
• Light has wavelike properties
• Car (with good shock absorbers) on bumpy road
• Length of day across the year
• In fact, most small amplitude oscillations.

Frequency filters

Sieves are frequency filters:

• Large objects trapped in mesh
• Small objects pass through
• This allows us to keep only the big stuff or the small stuff.
• We can also use to sieves with different sized mesh to select only medium sized objects.

Filtering

Consider signal with 3 frequencies (15 Hz,41 Hz and 358 Hz):

Low Pass

High Pass

Notch Filter

Frequency filters

• Fourier Transforms extract frequency components from complex signals
• Human ear and eye perception work this way

Signal

FT

Speakers

Components

What the? Where the?

• Motion:
• Retina: M Ganglion Cells
• LGN: Magno Layers
• Cortex: Dorsal
• Color/Form
• Retina: P Ganglion Cells
• LGN: Parvo Layers
• Cortex: Ventral

Sine wave gratings

• Amplitude (contrast)
• Frequency (cycles/degrees)
• Phase (starting point)
• Orientation (rotation)

Receptive Fields

Hubel described retinal ganglion cells.

• On center, off surround excited by central light, inhibited by peripheral light, when both center and surround stimulated a subtle response on onset and offset
• Center off, on surround showed opposite pattern.
• Size of these fields is pretty small. At most center is about 1 degree.
• Each of these cells tuned to a particular size.

+

-

Hubel and Wiesel

• This video shows cells that show both on center and off center effects, as well as higher level cells that show orientation selection.
• Recordings from single cells (noisy, but reliable as a population)

Spatial Filtering

→ Ganglion cells act as a spatial filter

Center facilitation, Surround inhibition

2D on center off surround

1D on center off surround

Acuity and Resolution

• Photoreceptor resolution (i.e. connectivity pattern between photoreceptor and ganglion cell) sets a limit on our acuity.
• Receptive field must be smaller than ½ the period of the sine wave to perceive the texture.

Scene

Percept

Spatial Filtering

On_Center Off_Surround often described as mexican hat or sombrero, acts as a spatial filter

Therefore, we should be optimally sensitive to spatial frequencies typical for the wiring of our ganglion cells.

Edge enhancement

• On-center, off surround filters exaggerate edges: regions where brightness changes.
• Regions of constant brightness (all dark or all bright) of little importance.
• Analogy: Caricatures often easier to recognize because they exaggerate defining features.

Spatial Filtering

On_Center Off_Surround often described as mexican hat or sombrero, acts as a spatial filter

Therefore, we should be optimally sensitive to spatial frequencies typical for the wiring of our ganglion cells.

Spatial Filtering

Ganglion cells act as a spatial filter

A receptive field has ‘goldilocks’ frequency when it has optimal response.

Less signal

Less signal

Optimal signal

Spatial Filtering

Phase influences response

• Consider grating with ganglion cells’ optimal frequency.
• Very strong response if receptive field centered on bright band.
• Inhibited response if centered on dark band.
• Across cells: edge detection

Contrast and Spatial Frequency

• Specific cells have receptive fields that tune them to seen spots of a certain size.
• What sizes is the human brain optimally good at detecting?
• We can test contrast sensitivity across spatial frequencies to find out.

​

Contrast

Spatial Frequency

Sensitivity to spatial frequency

Humans tuned for specific frequencies

reduce contrast

reduce frequency

Visible

Invisible

Contrast

Spatial Frequency

Lasik

• Different lighting conditions (high, medium, low) change contrast sensitivity function.
• Lasik surgery can change your contrast sensitivity function.

Hybrid images

• Why does Einstein change when you squint?
• Squinting applies a low frequency filter (allows only low frequencies through)
• “Behind every great man is a great woman”

Hybrid images

Mona Lisa

Secret smile

• Compare smile when you look directly at Mona Lisa versus see her from the corner of your eye.
• Remember, sensitivity to spatial frequencies changes with eccentricity.
• Sensitivity to spatial frequencies change.

La Bella Principessa (c. 1496)

Secret smile

• This painting predates the Mona Lisa.
• Young lady seems to be smiling, from others, the smile appears to have vanished
• Conducted experiment to prove this.
• Did Leo do it on purpose!?

Celebrity Face Illusion

Tilt illusions

• Our perception of grating orientation influenced by their surround.
• We can see after effects of fatigue and habituation by adapting to one stimuli and then looking at another.
• At least some of this happens in the brain, not retina: we can adapt left eye and see illusion in right eye.

From retina to brain

Optic nerve sends signals to brain

• Optic chiasm: visual fields
• Superior colliculus: eye movements
• Thalamus (Lateral geniculate Nucleus, LGN)
• From LGN to cortex

Lateral Geniculate Nucleus

• The Thalamus acts as relay station from senses to the cortex
• Distinct regions for each sense.
• LGN is one of the thalamic ‘nuclei’.
• It handles visual information

​

LGN

• 6 Layers
• Layers 1,2 magnocellular, monochromatic.
• Layers 4-6 parvocellular, color maintained.
• Connects to primary visual cortex.

​

​

Cortical Topography and Magnification

Retinotopic Map

LGN and V1 maintain retinotopic maps

• Spatial coordinates preserved
• Cortical magnification: Fovea heavily represented.

Retinotopic mapping

• Spatial location on retina preserved in LGN and primary visual cortex.
• *HUGE* representation for fovea, reflects huge number of cells covering central vision and reflects retinal/cortical magnification.

Human brain using fMRI

​

​

Tootell et al. (1982) monkey 2-deoxyglucose map

​

​

V1

• Primary visual cortex, aka: striate cortex, V1.
• Follows calcarine fissure.

Cortical magnification

• Acuity drops off away from retina

Entire visual system from receptors to cortex emphasize central vision.

V1 Orientation Tuning

• Hubel and Wiesel (1962) discovered that V1 simple cells have receptive fields that are tuned for specific orientations.
• By integrating across LGN cells, V1 performs edge and stripe detection.

​

Simple, complex, hypercomplex cells

Hubel & Wiesel http://en.wikipedia.org/wiki/Hypercomplex_cell

• Kuffler (JHU, late '50s) described cells with on center, off surround in cat retina. Cat was a lucky choice, as they do not have motion wiring of frog and rabbit or color complexity of monkey.
• His PhD students explored visual cortex of cat
• Initially they failed to find on center-off surround.
• Serendipitous discovery: inserting slide into projector
• Cell responded to direction of motion (complex cell)

​

Simple

Complex

Hypercomplex cells

H&W defined hypercomplex cells that have not only preferred orientation but also end stopping. Preferentially selecting lines of a specific length.

Simple, complex, hypercomplex cells

Ocular dominance

• 1960s - also Hubel and Wiesel
• Nobel Prize
• In LGN cells respond to either one eye or the other.
• In V1, preferential activation for one eye. Clear pattern to ocular dominance.

Cortical Hypercolumn

...given the proposed ice cube model of 1 mm² of visual cortex, and the observation of the behaviour of visual field coverage, it may be considered that a given 1 mm² of visual cortex contains all the cortical machinery required to process visual information in all possible ways for a given point in visual space

-Hubel and Wiesel

V1 organization

In addition to retinotopic map.

It also has a map for orientation, with neighboring columns interested in similar edge orientations.

Development of contrast sensitivity

• Perception of visual contrasts develops across years.
• Note that at a very young ages, sensitivity peaks at lower frequencies than at older ages.

Clinical considerations

• Critical window of development. Young visual system needs exposure to wire itself.
• For example, untreated childhood cataracts, strabismus (misalignment) and anisometropia (unequal refraction) can lead to amblyopia (lazy eye) with poor stereo depth perception.

Visual illusions

• Frasers’ spiral: Actually

series of concentric circles

due to alignment of threads.

• Zöllner illusion: long parallel lines appear misaligned due to angles of flanking short lines.
• Poggendorf illusion: blue line looks shifted.
• Reveal how perception is an interpretation of reality.

Cafe wall illusion

• Shifted chessboard: parallel lines appear tilted if tiles out of phase.
• Effect enhanced by ‘mortar’ between lines.