Perceptual Development I: Methods, Vision, and Face Perception Study of Constancy, and Face Perception Notes on Face Perception

Methods for Studying Infant Perception

There are several specialized techniques used to understand how infants perceive their environment, as they cannot provide verbal reports of their experiences.

The Preference Method

• Procedure: This method involves presenting multiple stimuli simultaneously to the infant. A ‘blind’ observer, who does not know which stimulus is which to prevent bias, records the precise amount of time the infant spends looking at each stimulus.

• Interpretation: A longer looking time is used as an indicator that the stimulus is ‘preferred’ by the infant.

• Limitations: If no preference is shown (equal looking time), the results are ambiguous. It could mean a failure to discriminate between the stimuli, or it could mean both stimuli are equally interesting to the infant.

• Historical Context: This method was famously utilized by Fantz ($1964$).

The Habituation Method

• The Boredom Principle: This method relies on the fact that infants lose interest in a stimulus after seeing it repeatedly.

• Apparatus: Often uses a pacifier containing electrical circuitry to measure sucking rate and pressure as an indicator of interest/arousal.

• Familiarization Phase: The same stimulus is presented repeatedly to the infant until they stop paying attention (habituation).

• Test Phase: A novel stimulus is presented. If the infant’s response (e.g., sucking rate or looking time) increases toward the novelty, it indicates the infant has noticed a difference between the old and new stimuli.

Physiological Methods

• Autonomic Changes: Researchers measure changes in heart rate or breathing rate; these fluctuations indicate that a change in the environment has been perceived.

• Evoked Potentials: This involves measuring changes in the pattern of electrical activity in the brain to confirm a stimulus has been detected.

• Specific Brain Regions:

  • Visual detection: Associated with activity in the occipital lobes.

  • Auditory detection: Associated with activity in the temporal lobes.

Visual Perception: Acuity and Early Development

Sensation versus Perception

• Sensation: The acquisition of information through the sensory organs (seeing, hearing, tasting, smelling, touching) and the transmission of that data to the brain.

• Perception: The attribution of meaning to those sensations. This includes recognizing objects, faces, depth, patterns, voices, and language.

Newborn Vision

• State at Birth: Vision is the least developed sense at birth.

• Visual Acuity: The ability to see fine detail is very poor.

  • At birth: Acuity is approximately $20/600$, which is $30$ times worse than an average adult.

  • At $6$ months: It improves to $20/100$, which is $5$ times worse than an adult.

  • At $1$ year: Acuity reaches adult levels ($20/20$).

• Accommodation: The ability of the eye to focus on objects at various distances is poor in newborns.

• Color Vision: Initially, infants have difficulty discriminating blues, greens, and yellows from white. By $2$ to $3$ months, they can discriminate all basic colors.

The Impact of Early Visual Deprivation

• Experience Matters: Early visual input is critical for development.

• Congenital Cataracts Study (Maurer, Lewis, Brent, & Levin, $1999$): Researchers compared infants born with dense central cataracts to age-matched controls. Infants were tested at four points after corrective lens insertion: $10$ minutes, $1$ hour, $1$ week, and $1$ month.

  • At $10$ minutes (before significant visual input), acuity was still significantly worse.

  • Significant improvements occurred after $1$ hour, $1$ week, and $1$ month of input.

  • However, visual acuity does not reach normal adult levels even if bilateral cataracts are removed early.

• Long-term Deficits (Le Grand et al., $2001$): Infants whose cataracts were surgically corrected between $2$ and $6$ months of age were tested $9$ years later. They showed significant deficits in spatial feature discrimination (sensing the spacing between facial features) compared to controls.

Development of Patterns, Form, and Constancies

Patterns and Form

• $0$ to $2$ Months: Infants prefer stimuli they can see clearly, characterized as moderately complex, high contrast, and moving (Fantz, $1961$).

• $2$ to $3$ Months: Vision improves, allowing infants to explore patterns in detail (Bronson, $1994$). They shift from simply looking at a stimulus to identifying "what" an object is.

Shape Constancy

• Definition: Perceiving an object as the same shape despite changes in its orientation or the angle from which it is viewed.

• Evidence (Slater & Morison, $1985$): Newborns were habituated to one of two different-sized objects shown at different angles set to produce the same retinal image (e.g., a large trapezium at $60^{\circ}$ looking like a square in the frontal plane). Preferential looking tasks showed newborns are sensitive to the real shape, indicating shape constancy is present at birth.

Size Constancy

• Definition: Perceiving an object as the same size despite changes in the size of the retinal image as it moves closer or further away.

• Evidence (Slater, Mattock, & Brown, $1990$): Newborns were habituated to one of two different-sized objects at distances calculated to produce identical retinal sizes (a small object near vs. a large object far). Infants demonstrated sensitivity to the real size of the cube, indicating size constancy at birth.

Object Unity and Trajectory

• Object Unity (Kellman & Spelke, $1983$): $4$-month-olds habituated to a rod moving behind a block perceive it as one continuous rod rather than two pieces. However, newborns do not show this perception (Slater et al., $1996$).

• Object Trajectory (Johnson et al., $2003$):

  • $2$ months: Perceive trajectories as discontinuous.

  • $4$ months: Perceive continuity only if the occluder is narrow or the time hidden is short.

  • $6$ months: Perceive full trajectory continuity.

Face Perception

The Importance of Faces

Faces are one of the most critical stimuli in an infant's world; attention to them is advantageous for both survival and learning. There is an ongoing debate regarding whether the preference for faces is innate.

Evidence for Innate Bias

• Newborn Preferences: Newborns (even those less than $1$ hour old) prefer face-configured stimuli over scrambled or blank stimuli, specifically tracking moving faces more (Johnson et al., $1991$; Turati, $2004$).

• Static and Schematic Stimuli: Preferences extend to static images and schematic representations.

• Familiar Faces:

  • Newborns ($1$ to $2$ days old) will suck on a dummy at a specific rate to see a photo of their mother over a stranger (Walton et al., $1992$).

  • Caveat: This recognition of the mother disappears if her hairline is covered (Pascalis et al., $1995$).

Preference for General Visual Characteristics

Some argue face preference is actually a preference for certain visual properties:

• Up-Down Asymmetry (Simion et al., $2002$): Newborns ($1$ to $4$ days old) prefer stimuli with more elements in the upper half.

• Non-Face Configurations: Turati et al. ($2002$) found that newborns show a preference for up-down asymmetry even in non-face-like configurations, suggesting face processing might not be distinct from general visual processing (e.g., preference for curved contours and top-heavy patterns).

Developmental Tuning of Face Processing

• Attractiveness (Langlois et al., $1991$): By $3$ to $6$ months, infants prefer "attractive" faces. These are often "prototype" or "average" faces created by merging many different images.

• Species-Specific Tuning (Pascalis et al., $2002$):

  • At $6$ months, infants can discriminate between individual human faces and individual monkey faces.

  • By $9$ months, infants lose the ability to discriminate between individual monkey faces, while retaining the ability for human faces.

Theoretical Accounts

1. Stimulus Characteristics (e.g., Turati, $2004$): Faces are appealing because they are high contrast, "top-heavy" (more elements in the top half), and dynamic.

2. Innate Mechanism (Morton & Johnson, $1991$): Infants possess an innate face-detecting brain mechanism that directs attention to face-like configurations.

• Synthesis: Regardless of the underlying cause, these mechanisms ensure newborns attend to faces, providing the visual input necessary to develop a sophisticated face-processing system.