I'm new to neuroscience research and came across a term called negative spikes (sometimes also used in conjunction with terms biphasic spikes or positive-negative spikes), but could not find what it meant from scientific papers. Can a spike occur in a negative direction (hyperpolarization from the resting potential first, followed by depolarization) or does this term mean something else entirely?
Intracellular recordings of action potentials always show the same format with which you are familiar. There is polarisation which the stimulus reduces, a threshold is reached, there is a positive spike, then repolarisation, hyperpolarisation, a refractory period an a return to the baseline polarisation.
The term "negative spike" applies to extracellular recordings. These measure the potential between a point close to but outside the cell and a reference point. The inflow of cations into the cell causes a drop in extracellular potential, which appears as a negative spike in the recording.