Biological psychology, sometimes referred to as psychobiology or biopsychology, is a subfield of psychology. Psychology may be defined as the scientific study of mental processes and behavior. Biological psychologists most commonly use an experimental approach to the study of psychology; by definition, the experimental manipulation is usually a biological one and as such contributes to evidence based practice. Alternatively, biological psychologists may be interested in measuring some biological variable (e.g., an anatomical, physiological, or genetic variable) in an attempt to relate it quantitatively or qualitatively to a psychological (often behavioral) variable.
In many cases, humans may serve as experimental subjects in biological psychology experiments; however, a great deal of the experimental literature in biological psychology comes from the study of non-human species, most frequently rats, mice, and monkeys. As a result, a critical assumption in biological psychology is that organisms share biological and behavioral similarities, enough to permit extrapolations across species. This allies biological psychology closely with comparative psychology, evolutionary psychology, and evolutionary biology. Biological psychology also has paradigmatic and methodological similarities to neuropsychology, which relies heavily on the study of the behavior of humans with nervous system dysfunction (i.e., a non-experimentally based biological manipulation).

Synonyms for biological psychology include biopsychology, behavioral neuroscience, and psychobiology. Physiological psychology is another term often used synonymously with biological psychology, though some authors would make physiological psychology a subfield of biological psychology, with an appropriately more narrow definition.
The distinguishing characteristic of a biological psychology experiment is that either the independent variable of the experiment is biological, or some dependent variable is biological. In other words, the nervous system of the organism under study is permanently or temporarily altered, or some aspect of the nervous system is measured (usually to be related to a behavioral variable). Most commonly, these manipulations and measurements concern non-human subjects.

Disabling or Decreasing Neural Function
Lesions - A classic method in which a brain-region of interest is enabled. Lesions can be placed with relatively high accuracy thanks to a variety of brain 'atlases' which provide a map of brain regions in 3-dimensional stereotactic coordinates.
Electrolytic lesions - Neural tissue is destroyed by the use of electric run through.
Chemical lesions - Neural tissue is destroyed by the infusion of a neurotoxin.
Temporary lesions - Neural tissue is temporarily disabled by cooling or by the use of anesthetics such as tetrodotoxin.
Transcranial magnetic stimulation - A new technique usually used with human subjects in which a magnetic coil applied to the scalp causes unsystematic electrical activity in nearby cortical neurons which can be experimentally analyzed as a functional lesion.
Psychopharmacological manipulations - A chemical receptor antagonist enduces neural activity by interfering with neurotransmission. Antagonists can be delivered systemically (such as by intravenous injection) or locally (intracebrally) during a surgical procedure.

Enhancing Neural Function
Electrical Stimulation - A classic method in which neural activity is enhanced by application of a small electrical current (too small to cause significant cell death).
Psychopharmacological manipulations - A chemical receptor agonist facilitates neural activity by enhancing or replacing endogenous neurotransmitters. Agonists can be delivered systemically (such as by intravenous injection) or locally (intracebrally) during a surgical procedure.
Transcranial magnetic stimulation - In some cases (for example, studies of motor cortex, this technique can be analyzed as having a stimulatory effect (rather than as a functional lesion) .

Measuring Neural Activity
Single unit recording - The measurement of the electrical activity of one neuron, often in the context of an ongoing behavioral (psychological) task.
Multielectrode recording - The use of a bundle of fine electrodes to record the simultaneous activity of up to hundreds of neurons.
fMRI - Functional magnetic resonance imaging, a technique most frequently applied on human subjects, in which changes in cerebral blood flow can be detected in an MRI apparatus and are taken to indicate relative activity of larger scale brain regions (i.e., on the order of hundreds of thousands of neurons).
Electroencephalography - Or EEG; and the derivative technique of event-related potentials, in which scalp electrodes monitor the average activity of neurons in the cortex (again, used most frequently with human subjects).
Functional neuroanatomy - In which the expression of some anatomical marker is taken to reflect neural activity. For example, the expression of immediate early genes is thought to be caused by vigorous neural activity. Likewise, the injection of 2-deoxyglucose prior to some behavioral task can be followed by anatomical localization of that chemical; it is taken up by neurons that are electrically active.

Genetic Manipulations
QTL mapping - The influence of a gene in some behavior can be statistically inferred by studying inbred strains of some species, most commonly mice. The recent sequencing of the genome of many species, most notably mice, has facilitated this technique.
Selective breeding - Organisms, often mice, may be bred selectively among inbred strains to create a recombinant congenic strain. This might be done to isolate an experimentally interesting stretch of DNA derived from one strain on the background genome of another strain to allow stronger inferences about the role of that stretch of DNA.
Genetic engineering - The genome may also be experimentally-manipulated; for example, knockout mice can be engineered to lack a particular gene, or a gene may be expressed in a strain which does not normally do so (the 'knock in'). Advanced techniques may also permit the expression or suppression of a gene to occur by injection of some regulating chemical.

Topic Areas In Biological Psychology
In general, biological psychologists study the same issues as academic psychologists, though limited by the need to use nonhuman species. As a result, the bulk of literature in biological psychology deals with mental processes and behaviors that are shared across mammalian species, such as:

Sensation and perception
Motivated behavior (hunger, thirst, sex)
Control of movement
Learning and memory
Sleep and biological rhythms
Emotion
However, with increasing technical sophistication and with the development of more precise noninvasive methods that can be applied to human subjects, biological psychologists are beginning to contribute to other classical topic areas of psychology, such as:

Language
Reasoning and decision making
Consciousness
Biological psychology has also had a strong history of contributing to medical disorders including those that fall under the purview of clinical psychology and psychopathology (also known as abnormal psychology). Although animal models for all mental illnesses do not exist, the field has contributed important therapeutic data on a variety of conditions, including:
Parkinson's Disease
Huntington's Disease
Alzheimer's Disease
Clinical depression
Schizophrenia
Autism
Anxiety
Drug abuse, including alcoholism

Philosophical Implications
Biopsychology is related to the philosophical problem of mental causation through the common premise that all mental and psychological phenomena supervene on material structures and processes. As an alternative to this premise, it has been suggested (for example) that the brain may be sensitive to quantum fluctuations, and that this may exempt it from strict physical and biological determinacy. However, such ideas remain speculative, as is the more widespread assumption that some combination of physical and biological causation accounts for all mental and behavioral phenomena.